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.

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.