diff --git a/README.md b/README.md
index 473ba0c..a2c66c6 100644
--- a/README.md
+++ b/README.md
@@ -1,7 +1,7 @@
 # HTTP resource pack server
 
 A common scenario is that you have a set of static resources that you want to
-server up quickly via HTTP (for example: stylesheets, WASM).
+serve up quickly via HTTP (for example: stylesheets, WASM).
 
 This package provides a `net/http`-compatible `http.Handler` to do so, with
 support for:
@@ -20,4 +20,3 @@ The workflow is as follows:
 Only the minimal header processing necessary for correctness (Content-Length,
 etc.) is carried out by `htpack.Handler`; the handler can be combined with
 middleware for further processing (adding headers, `http.StripPrefix`, etc.).
-
diff --git a/cmd/htpacker/inspector.go b/cmd/htpacker/inspector.go
new file mode 100644
index 0000000..c89130f
--- /dev/null
+++ b/cmd/htpacker/inspector.go
@@ -0,0 +1,60 @@
+package main
+
+import (
+	"fmt"
+	"os"
+
+	"github.com/lwithers/htpack/internal/packed"
+)
+
+// Inspect a packfile.
+//  TODO: verify etag; verify integrity of compressed data.
+//  TODO: skip Gzip/Brotli if not present; print ratio.
+func Inspect(filename string) error {
+	f, err := os.Open(filename)
+	if err != nil {
+		return err
+	}
+	defer f.Close()
+
+	hdr, dir, err := packed.Load(f)
+	if hdr != nil {
+		fmt.Printf("Header: %#v\n", hdr)
+	}
+	if dir != nil {
+		fmt.Printf("%d files:\n", len(dir.Files))
+		for path, info := range dir.Files {
+			fmt.Printf(" • %s\n"+
+				"    · Etag:         %s\n"+
+				"    · Content type: %s\n"+
+				"    · Uncompressed: %s (offset %d)\n"+
+				"    · Gzipped:      %s (offset %d)\n"+
+				"    · Brotli:       %s (offset %d)\n",
+				path, info.Etag, info.ContentType,
+				printSize(info.Uncompressed.Length), info.Uncompressed.Offset,
+				printSize(info.Gzip.Length), info.Gzip.Offset,
+				printSize(info.Brotli.Length), info.Brotli.Offset,
+			)
+		}
+	}
+	return err
+}
+
+func printSize(size uint64) string {
+	switch {
+	case size < 1<<10:
+		return fmt.Sprintf("%d bytes", size)
+	case size < 1<<15:
+		return fmt.Sprintf("%.2f KiB", float64(size)/(1<<10))
+	case size < 1<<20:
+		return fmt.Sprintf("%.1f KiB", float64(size)/(1<<10))
+	case size < 1<<25:
+		return fmt.Sprintf("%.2f MiB", float64(size)/(1<<20))
+	case size < 1<<30:
+		return fmt.Sprintf("%.1f MiB", float64(size)/(1<<20))
+	case size < 1<<35:
+		return fmt.Sprintf("%.2f GiB", float64(size)/(1<<30))
+	default:
+		return fmt.Sprintf("%.1f GiB", float64(size)/(1<<30))
+	}
+}
diff --git a/cmd/htpacker/packer.go b/cmd/htpacker/packer.go
index 5a6ae74..213f938 100644
--- a/cmd/htpacker/packer.go
+++ b/cmd/htpacker/packer.go
@@ -37,6 +37,7 @@ type packInfo struct {
 	offset, len uint64
 }
 
+// Pack a file.
 func Pack(filesToPack FilesToPack, outputFilename string) error {
 	finalFname, outputFile, err := writefile.New(outputFilename)
 	if err != nil {
@@ -47,8 +48,8 @@ func Pack(filesToPack FilesToPack, outputFilename string) error {
 
 	// write initial header (will rewrite offset/length when known)
 	hdr := &packed.Header{
-		Magic:           123,
-		Version:         1,
+		Magic:           packed.Magic,
+		Version:         packed.VersionInitial,
 		DirectoryOffset: 1,
 		DirectoryLength: 1,
 	}
@@ -117,7 +118,6 @@ func packOne(packer *packWriter, fileToPack FileToPack) (info packed.File, err e
 	}
 	defer unix.Munmap(data)
 
-	// TODO: content-type, etag
 	info.Etag = etag(data)
 	info.ContentType = fileToPack.ContentType
 	if info.ContentType == "" {
@@ -283,6 +283,8 @@ func (pw *packWriter) CopyFrom(in *os.File) (uint64, error) {
 		return 0, pw.err
 	}
 
+	fmt.Fprintf(os.Stderr, "[DEBUG] in size=%d\n", fi.Size())
+
 	var off int64
 	remain := fi.Size()
 	for remain > 0 {
@@ -294,8 +296,9 @@ func (pw *packWriter) CopyFrom(in *os.File) (uint64, error) {
 		}
 
 		amt, err = unix.Sendfile(int(pw.f.Fd()), int(in.Fd()), &off, amt)
+		fmt.Fprintf(os.Stderr, "[DEBUG]   sendfile=%d [off now %d]\n", amt, off)
 		remain -= int64(amt)
-		off += int64(amt)
+		//off += int64(amt)
 		if err != nil {
 			pw.err = err
 			return uint64(off), pw.err
diff --git a/cmd/htpacker/quick_pack.go b/cmd/htpacker/quick_pack.go
index 7d33323..f851e15 100644
--- a/cmd/htpacker/quick_pack.go
+++ b/cmd/htpacker/quick_pack.go
@@ -9,13 +9,14 @@ import (
 )
 
 func main() {
-	if err := run(); err != nil {
+	//if err := dopack(); err != nil {
+	if err := Inspect("out.htpack"); err != nil {
 		fmt.Fprintln(os.Stderr, err)
 		os.Exit(1)
 	}
 }
 
-func run() error {
+func dopack() error {
 	raw, err := ioutil.ReadFile("in.yaml")
 	if err != nil {
 		return err
diff --git a/internal/packed/load.go b/internal/packed/load.go
new file mode 100644
index 0000000..10609e5
--- /dev/null
+++ b/internal/packed/load.go
@@ -0,0 +1,304 @@
+package packed
+
+import (
+	fmt "fmt"
+	"os"
+	"path"
+	"strings"
+)
+
+const (
+	// Magic number to identify .htpack files.
+	Magic = 0xb6e61a4b415ed33b
+
+	// VersionInitial is the version number used by the initial packed
+	// format. Loading a file with a higher version number will cause an
+	// error to be returned.
+	VersionInitial = 1
+)
+
+// Load a ready-packed file.
+func Load(f *os.File) (*Header, *Directory, error) {
+	hdr, err := loadHeader(f)
+	if err != nil {
+		return nil, nil, err
+	}
+
+	dir, err := loadDirectory(f, hdr)
+	if le, ok := err.(*LoadError); ok {
+		// augment error
+		le.Magic = hdr.Magic
+		le.Version = hdr.Version
+	}
+	return hdr, dir, err // we may have a partial dir
+}
+
+// loadHeader retrieves and decodes the header from the start of the file. It
+// ensures the magic number and the version number match. Errors are returned
+// as type LoadError.
+func loadHeader(f *os.File) (*Header, error) {
+	raw := make([]byte, 36)
+	if _, err := f.ReadAt(raw, 0); err != nil {
+		return nil, &LoadError{
+			Cause:      IOError,
+			Underlying: err,
+		}
+	}
+
+	hdr := new(Header)
+	if err := hdr.Unmarshal(raw); err != nil {
+		return nil, &LoadError{
+			Cause:      HeaderUnmarshalError,
+			Underlying: err,
+		}
+	}
+
+	switch {
+	case hdr.Magic != Magic:
+		return nil, &LoadError{
+			Cause:   MagicMismatch,
+			Magic:   hdr.Magic,
+			Version: hdr.Version,
+		}
+	case hdr.Version < VersionInitial:
+		return nil, &LoadError{
+			Cause:   VersionTooOld,
+			Magic:   hdr.Magic,
+			Version: hdr.Version,
+		}
+	case hdr.Version > VersionInitial:
+		return nil, &LoadError{
+			Cause:   VersionTooNew,
+			Magic:   hdr.Magic,
+			Version: hdr.Version,
+		}
+	}
+
+	return hdr, nil
+}
+
+// loadDirectory reads the directory from a file. The directory is checked
+// for consistency (offsets, filenames) but not integrity (file data is not
+// read/checksummed).
+func loadDirectory(f *os.File, hdr *Header) (*Directory, error) {
+	fi, err := f.Stat()
+	if err != nil {
+		return nil, &LoadError{
+			Cause:      IOError,
+			Underlying: err,
+		}
+	}
+	fileSize := uint64(fi.Size())
+
+	if hdr.DirectoryOffset+hdr.DirectoryLength > fileSize {
+		return nil, &LoadError{
+			Cause: BadOffsetError,
+		}
+	}
+
+	raw := make([]byte, hdr.DirectoryLength)
+	if _, err := f.ReadAt(raw, int64(hdr.DirectoryOffset)); err != nil {
+		return nil, &LoadError{
+			Cause:      IOError,
+			Underlying: err,
+		}
+	}
+
+	dir := new(Directory)
+	if err := dir.Unmarshal(raw); err != nil {
+		return nil, &LoadError{
+			Cause:      DirectoryUnmarshalError,
+			Underlying: err,
+		}
+	}
+
+	return dir, checkDirectory(dir, fileSize)
+}
+
+// checkDirectory verifies the consistency of the htpack file (offsets,
+// filenames). It does not verify integrity (checksums).
+func checkDirectory(dir *Directory, fileSize uint64) error {
+	files := map[string]struct{}{}
+
+	for filename, info := range dir.Files {
+		var err error
+
+		// validate filename (not duplicate, canonical, etc)
+		if _, dup := files[filename]; dup {
+			err = fmt.Errorf("duplicate path %q", filename)
+		}
+		files[filename] = struct{}{}
+
+		if !path.IsAbs(filename) {
+			err = fmt.Errorf("relative path %q", filename)
+		}
+		if path.Clean(filename) != filename {
+			err = fmt.Errorf("non-canonical path %q", filename)
+		}
+		if err != nil {
+			return &LoadError{
+				Cause:      InvalidPath,
+				Underlying: err,
+				Path:       filename,
+			}
+		}
+
+		// ensure uncompressed data is present
+		if info.Uncompressed == nil {
+			return &LoadError{
+				Cause: MissingUncompressed,
+				Path:  filename,
+			}
+		}
+
+		// validate offsets
+		checkOffset(&err, filename, info.Uncompressed, fileSize)
+		checkOffset(&err, filename, info.Gzip, fileSize)
+		checkOffset(&err, filename, info.Brotli, fileSize)
+		if err != nil {
+			return &LoadError{
+				Cause: BadOffsetError,
+			}
+		}
+	}
+
+	return nil
+}
+
+func checkOffset(perr *error, filename string, data *FileData, fileSize uint64) {
+	if *perr != nil || data == nil {
+		return
+	}
+	if data.Offset+data.Length > fileSize {
+		*perr = &LoadError{
+			Cause: BadOffsetError,
+			Path:  filename,
+		}
+	}
+}
+
+// LoadError reports a problem interpreting the header of a pack file.
+type LoadError struct {
+	// Cause of the error.
+	Cause ErrorCause
+
+	// Underlying may be set if there is some more information about the
+	// error (e.g. I/O error).
+	Underlying error
+
+	// Magic as read from the file.
+	Magic uint64
+
+	// Version as read from the file.
+	Version uint64
+
+	// Path of an individual file within the pack, if relevant.
+	Path string
+}
+
+// ErrorCause enumerates the possible reasons for failure.
+type ErrorCause int
+
+const (
+	// HeaderUnmarshalError means we could not unmarshal the protobuf object
+	// at the head of the file.
+	HeaderUnmarshalError ErrorCause = iota
+
+	// DirectoryUnmarshalError means we could not unmarshal the protobuf
+	// object holding the directory contents.
+	DirectoryUnmarshalError
+
+	// IOError indicates we could not read a protobuf object.
+	IOError
+
+	// MagicMismatch occurs if the recorded magic number does not match
+	// the well-known constant Magic.
+	MagicMismatch
+
+	// VersionTooNew indicates that the file has a version number ahead of
+	// what this package can parse.
+	VersionTooNew
+
+	// VersionTooOld indicates that the file has a version number older than
+	// thisk package can parse.
+	VersionTooOld
+
+	// BadOffsetError means that the header or directory has indicated a
+	// position that lies outside of the file.
+	BadOffsetError
+
+	// InvalidPath is returned for a duplicate or otherwise invalid path.
+	// Underlying is set to a free-form string error describing the path.
+	InvalidPath
+
+	// MissingUncompressed indicates that a file in the pack does not have
+	// an uncompressed version present, which is mandatory.
+	MissingUncompressed
+)
+
+// Desc returns a description of the error cause.
+func (le *LoadError) Desc() string {
+	switch le.Cause {
+	case HeaderUnmarshalError:
+		return "not a .htpack file (header not valid packed.Header)"
+	case DirectoryUnmarshalError:
+		return "file corrupt (directory not valid packed.Directory)"
+	case IOError:
+		return "error reading from file"
+	case MagicMismatch:
+		return "magic number does not match"
+	case VersionTooNew:
+		return "version too new"
+	case VersionTooOld:
+		return "version too old"
+	case BadOffsetError:
+		return "file corrupt/truncated (offset past end of file)"
+	case InvalidPath:
+		return "filename invalid"
+	case MissingUncompressed:
+		return "missing uncompressed version"
+	default:
+		return "unknown error"
+	}
+}
+
+// Error returns a concise description of the error.
+func (le *LoadError) Error() string {
+	var b strings.Builder
+
+	b.WriteString(le.Desc())
+
+	var underlying, magic, version, path bool
+	switch le.Cause {
+	case HeaderUnmarshalError, IOError:
+		underlying = true
+	case MagicMismatch:
+		magic = true
+	case VersionTooNew, VersionTooOld:
+		version = true
+	case BadOffsetError:
+		path = le.Path != ""
+	case InvalidPath, MissingUncompressed:
+		path = true
+	}
+
+	if underlying {
+		b.WriteString(" [")
+		b.WriteString(le.Underlying.Error())
+		b.WriteString("]")
+	}
+	if magic {
+		fmt.Fprintf(&b, " (found magic 0x%X, expected 0x%X)",
+			le.Magic, uint64(Magic))
+	}
+	if version {
+		fmt.Fprintf(&b, " (found version %d; oldest supported: "+
+			"%d, newest: %d",
+			le.Version, VersionInitial, VersionInitial)
+	}
+	if path {
+		fmt.Fprintf(&b, " (path %q)", le.Path)
+	}
+
+	return b.String()
+}