Fonts: Recommendation
If you plan to use a bold-style font in your book, include a bold type face when declaring embedded fonts. This prevents iOS from having to synthesize a bold font from a normal weight font face.
Audio: Recommendation
When embedding audio, make sure you specify the correct media-type for your audio files in the OPF (the expected media-type for m4a files is "audio/m4a"). If the media-type is incorrect, the audio will not play after customers have purchased the book. This issue will not be evident when testing locally prior to delivering to Apple Books.
Audio Encoding
Previous versions of this guide required that audio be encoded as stereo. However, mono recordings are accepted if stereo is not available.
Store Rebranding
Changed the name of iBooks and iBooks Store to Apple Books throughout, except the names of prior versions of this guide in Revision History and the names of prior versions of the iBooks app. Tags, attribute names, attribute values, and file names used in the XML with ibooks in them have not changed. The name of iBooks Author and the .ibooks extension have also not changed.