Bookmarks is what you want to use.
Add those lines to .rockbox/config.cfg:
autocreate bookmarks: recent only - on
autoupdate bookmarks: on
autoload bookmarks: ask
use most-recent-bookmarks: unique only
That will maintain a list of 10 recent bookmarks.
Also, while listening to an audiobook, go to the context menu -> Bookmarks->Create Bookmark.
That will create a persistent bookmark, that is being updated with the progress, if you stop playback.