The Fluid, Electrolyte And Acid-base Companion, 1999 (with 2011 updates?), by Sarah Faubel and Joel Topf, is an outstanding nephrology text and workbook. It is beautifully illustrated, and boasts an excellent blend of basic and clinical science. It is easily one of the finest medical books ever published. Best of all, the entire book can be download free of charge at Dr. Topf’s website.
I recommend it very highly to medical students and residents.