I was frustrated by the lack of built-in support for mp3-based audiobooks so I wrote this app. It lets you load as many audiobooks as you have memory for. It remembers where you are in each file of each book (and of course, which file you are listening to for each book).
It has standard transport keys plus the ability to enter HH:MM:SS to skip directly to.
Download Link: social.zune.net/redirect?type=phoneApp&id=b1cab14b-e91d-e011-9264-00237de2db9e