Currently I’m using Google Play books and there are few things I’m used to and can’t live without:

• turning pages by volume keys
• cloud syncing (doesn’t have to be their cloud, but implementing some cloud services would be nice, bonus points for S3 compatible)
• searching text in the book
• supports epub

I want to switch to something open source because I’m often using my S-Pen to turn pages and I’d like my book reader to support it too. I’m fine with contributing the code myself (as I’ve done for the comic book reader app I’m using), but if there already is one, it would save me some work.