In regards to books, I would recommend at least checking out Steve Krug's Don't Make Me Think, possibly some other design orientated books and then read up upon usability in general.
You may wish to find a usability consultant too, or look for someone who hasn't used your app before with experience in this area to give you a few pointers.
For example, take a look at: http://www.usabilitypost.com/ or http://www.uxbooth.com/ and http://useit.com/
You may wish to find a usability consultant too, or look for someone who hasn't used your app before with experience in this area to give you a few pointers.