Originally Posted by sideral
Autoresume works by remembering a resume position for each track. If your audiobook comprises multiple tracks, autoresume does not (yet) remember in which track you were. You have to navigate to the last-played track manually, which will then be resumed mid-track.
Update: I have started working on a feature to remember and highlight the last-played track for each album, which is intended to allow easy (bookmarkless) audiobook resume and works in conjunction with the autoresume feature. If you're interested, head over to http://www.rockbox.org/tracker/task/12054
to try the patch or leave a comment.