You can use Speak Screen with a variety of apps:
http://www.apple.com/accessibility/ios/
Or you can use a specialised screen reader app like Voice Dream Reader. If you go this route I believe you'll need to strip DRM from any DRM-encumbered books. Apprentice Alf's blog will tell you how to do that. So format doesn't really matter much, so long as it's not iBooks - you can buy from Kindle, Kobo, Google Play, wherever, then strip and convert to the appropriate format (usually epub).