iOS Audiobook player without iTunes media library
I have been trying to answer this question for myself for ages. I've only recently found the solution for me - SpeedUp Player Pro seems to have all of the audiobook functionality I need. The hightlights are: 


Speed up (up to TRUE 2.5x)
Load items from Dropbox
Adjustable Forward and Backward Skip
A/B Repeat (for studying)
Gestures
Functional free version with some features disabled.


The app is not rock-solid in terms of stability, but I expect it will get there as the developer continues to work on it.
The developer's name is mix1009 and there is a link to his/her website.