The best general medical textbook for information about renal diseases at an undergraduate level is Davidson's Textbook of Medicine (Churchill Livingstone). The chapter in the 18th edition is good but the 19th will be the best of all. We know this because we wrote it.
For transplantation and urology, Forrest's Principles and Practice of Surgery, 4th edition, Churchill Livingstone (in press) - Urology chapter by L.Stewart, and Transplantation chapter by J.Forsyth and H.Pleass.
Essential Urology, by Bullock, Sibley, and Whitaker, 2nd edition, Churchill Livingstone, 1994 - getting a bit old, but nice and straightforward.
Principles of Nephrology, ed J.Feehally and R.Johnson, Mosby, 2000 - goes into much more detail and is suitable for detailed study if you have a case to prepare, for example. It is really a postgraduate text, but it is a single volume and is excellent.
Transplantation - two books:
Companion to Specialist Surgical Practice Vol 7, Transplantation Surgery, Ed J.Forsyth, and Kidney Transplantation by P.J.Morris (3rd edition) - very big now, possibly too detailed.Pathology - if you want to go so far as to look something up in a pathology book, its probably worth going for the more complete Robbins, rather than a smaller one. The renal/ urology section in there is good. (Cottran, Kumar and Collins; Robbins' Pathological Basis of Disease.)
This website covers many important diseases in more than enough detail for undergraduate and most postgraduate purposes. If you are looking for a specific disease, go to EdRenINFO.
Our links page has a list of web resources - some are aimed at a postgraduate medical audience but some are highly relevant.
For urology, the NIDDK Urological information section has excellent patient-oriented information on urological topics. The Nephron Information Centre lists these and a long list of other excellent resources for doctors too.
At the stage medical students are moving around the specialities it is time to think about looking regularly at some of the following weekly journals:
British Medical Journal (BMJ) - good medical news, some reviews of topical subjects, but some sensationalist tendencies and a heavy (?too heavy) emphasis on evidence-based medicine. Consider starting from the back of the journal when looking at the paper copy. The full text of the BMJ is freely available on-line.
Lancet - medical news and short reviews, some high-impact papers, but a history of some quirky reviewing at times. Lancet website
New England Journal of Medicine - especially the clinicopathological conferences. Fewer, longer articles than the above, and a heavier read, but the place to publish the highest impact clinical research. NEJM website