Bookmark

Re: Why is there no way to bookmark an audiobook in Books for iOS?

Maybe bookmarks are for pages, or web-pages, and audiobooks have none.

Seems you choose to go to a specific time or chapter instead, in audiobooks.

Listen to audiobooks in Apple Books - Apple Support

Eg: says: "Control audio playback

Skip to a chapter, change the speed, set a sleep timer, and more"

Listen to audiobooks in Books on iPhone – Apple Support

Eg: says:

While the audiobook is playing:

"Go to a specific time: Drag the slider below the audiobook cover left or right".

or

"Go to a chapter: Tap , then tap a chapter.

Note: Some audiobooks refer to chapters as tracks, or don’t define chapters."

Hope this helps :-)