diff --git a/id.go b/id.go
index b2c8a0c..4670192 100644
--- a/id.go
+++ b/id.go
@@ -26,8 +26,20 @@ func Identify(r io.ReadSeeker) (format Format, fileType FileType, err error) {
 	case string(b[0:4]) == "OggS":
 		return VORBIS, OGG, nil
 
-	case string(b[4:11]) == "ftypM4A":
-		return AAC, MP4, nil
+	case string(b[4:8]) == "ftyp":
+		b = b[8:11]
+		fileType = UnknownFileType
+		switch string(b) {
+		case "M4A":
+			fileType = M4A
+
+		case "M4B":
+			fileType = M4B
+
+		case "M4P":
+			fileType = M4P
+		}
+		return MP4, fileType, nil
 
 	case string(b[0:3]) == "ID3":
 		b := b[3:]
diff --git a/mp4.go b/mp4.go
index 84f241d..be31261 100644
--- a/mp4.go
+++ b/mp4.go
@@ -57,12 +57,18 @@ func (f atomNames) Name(n string) []string {
 }
 
 // metadataMP4 is the implementation of Metadata for MP4 tag (atom) data.
-type metadataMP4 map[string]interface{}
+type metadataMP4 struct {
+	fileType FileType
+	data     map[string]interface{}
+}
 
 // ReadAtoms reads MP4 metadata atoms from the io.ReadSeeker into a Metadata, returning
 // non-nil error if there was a problem.
 func ReadAtoms(r io.ReadSeeker) (Metadata, error) {
-	m := make(metadataMP4)
+	m := metadataMP4{
+		data: make(map[string]interface{}),
+		fileType: UnknownFileType,
+	}
 	err := m.readAtoms(r)
 	return m, err
 }
@@ -137,8 +143,8 @@ func (m metadataMP4) readAtomData(r io.ReadSeeker, name string, size uint32) err
 	b = b[8:]
 
 	if name == "trkn" || name == "disk" {
-		m[name] = int(b[3])
-		m[name+"_count"] = int(b[5])
+		m.data[name] = int(b[3])
+		m.data[name+"_count"] = int(b[5])
 		return nil
 	}
 
@@ -157,7 +163,7 @@ func (m metadataMP4) readAtomData(r io.ReadSeeker, name string, size uint32) err
 			Data:     b,
 		}
 	}
-	m[name] = data
+	m.data[name] = data
 
 	return nil
 }
@@ -220,13 +226,13 @@ func readCustomAtom(r io.ReadSeeker, size uint32) (string, uint32, error) {
 }
 
 func (metadataMP4) Format() Format     { return MP4 }
-func (metadataMP4) FileType() FileType { return AAC }
+func (m metadataMP4) FileType() FileType { return m.fileType }
 
-func (m metadataMP4) Raw() map[string]interface{} { return m }
+func (m metadataMP4) Raw() map[string]interface{} { return m.data }
 
 func (m metadataMP4) getString(n []string) string {
 	for _, k := range n {
-		if x, ok := m[k]; ok {
+		if x, ok := m.data[k]; ok {
 			return x.(string)
 		}
 	}
@@ -235,7 +241,7 @@ func (m metadataMP4) getString(n []string) string {
 
 func (m metadataMP4) getInt(n []string) int {
 	for _, k := range n {
-		if x, ok := m[k]; ok {
+		if x, ok := m.data[k]; ok {
 			return x.(int)
 		}
 	}
@@ -277,7 +283,7 @@ func (m metadataMP4) Year() int {
 
 func (m metadataMP4) Track() (int, int) {
 	x := m.getInt([]string{"trkn"})
-	if n, ok := m["trkn_count"]; ok {
+	if n, ok := m.data["trkn_count"]; ok {
 		return x, n.(int)
 	}
 	return x, 0
@@ -285,14 +291,14 @@ func (m metadataMP4) Track() (int, int) {
 
 func (m metadataMP4) Disc() (int, int) {
 	x := m.getInt([]string{"disk"})
-	if n, ok := m["disk_count"]; ok {
+	if n, ok := m.data["disk_count"]; ok {
 		return x, n.(int)
 	}
 	return x, 0
 }
 
 func (m metadataMP4) Lyrics() string {
-	t, ok := m["\xa9lyr"]
+	t, ok := m.data["\xa9lyr"]
 	if !ok {
 		return ""
 	}
@@ -300,7 +306,7 @@ func (m metadataMP4) Lyrics() string {
 }
 
 func (m metadataMP4) Picture() *Picture {
-	v, ok := m["covr"]
+	v, ok := m.data["covr"]
 	if !ok {
 		return nil
 	}
diff --git a/tag.go b/tag.go
index 96a70b8..8e5afbc 100644
--- a/tag.go
+++ b/tag.go
@@ -73,7 +73,7 @@ const (
 	ID3v2_2              = "ID3v2.2" // ID3v2.2 tag format.
 	ID3v2_3              = "ID3v2.3" // ID3v2.3 tag format (most common).
 	ID3v2_4              = "ID3v2.4" // ID3v2.4 tag format.
-	MP4                  = "MP4"     // MP4 tag (atom) format.
+	MP4                  = "MP4"     // MP4 tag (atom) format (see http://www.ftyps.com/ for a full file type list)
 	VORBIS               = "VORBIS"  // Vorbis Comment tag format.
 )
 
@@ -86,7 +86,9 @@ type FileType string
 const (
 	UnknownFileType FileType = ""     // Unknown FileType.
 	MP3                      = "MP3"  // MP3 file
-	AAC                      = "AAC"  // M4A file (MP4)
+	M4A                      = "M4A"  // M4A file Apple iTunes (ACC) Audio
+	M4B                      = "M4B"  // M4A file Apple iTunes (ACC) Audio Book
+	M4P                      = "M4P"  // M4A file Apple iTunes (ACC) AES Protected Audio
 	ALAC                     = "ALAC" // Apple Lossless file FIXME: actually detect this
 	FLAC                     = "FLAC" // FLAC file
 	OGG                      = "OGG"  // OGG file