A much needed feature is the ability to "Return to Index". Given the very slow speed of page turns, the following real-world scenario is very annoying:
1. Browse Index
2. Click on a potentially interesting chapter.
3. Lose interest and want to browse for another potentially interesting chapter.
4. Return to index only to have to start from the beginning again.
Not sure about anyone here but I've got some epub formatted Oreilly books that contain 20 pages of chapter titles alone...