Apple Books supports EPUB (.epub) books that can be created with Pages or other third-party tools.
Apple Books supports the EPUB 3.3 specification, and is backwards compatible with previous versions of EPUB 3. The EPUB 3 specification be used to create flowing and fixed-layout books:
A flowing book is ideal for books that are primarily text content. A flowing book's text can be resized by the reader, and the text reflows to various screen sizes without requiring scaling of the content. Flowing books support images, audio, video, and interactivity.
A fixed-layout book can be created for books that need a precise design or layout. A fixed-layout book supports full-bleed images, audio, video, interactivity, and read aloud features. Read aloud features allow you to sync a narrator's voice with the text in the book.