A book can be styled using system fonts or embedded fonts. Both system fonts (fonts that exist on the device) and embedded fonts (developer supplied fonts within the book) are defined using standard CSS. Embedded fonts must be listed in the manifest of the OPF .opf. If you intend to provide embedded fonts in a flowing book or a fixed-layout book, you must set the "specified-fonts" option to true. See Defining Book Layout Metadata for an example.
Note: 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.
In flowing Books, if the "specified-fonts" property is set to true, the reader can select a new font when reading the book. However, readers always have the option in the Apple Books interface to return to the book's original fonts.
Note: Apple Books' fonts follow the font orientation guidelines of UTR (Unicode Technical Report) 50 draft 6.