Paul G. Schmitz (Saint Louis University TPD) recently published 2 new textbooks useful for students, residents, and fellows interested in kidney disorders. These are both FREE downloads from the iBooks store. These are both self-published and interactive (Macs, iPhones, or iPads for now). Content updates will also be free and will be performed by student, resident, and fellows.
Check them both out and provide Dr. Schmitz some feedback. They look fantastic.