Maurice Hinson has written a ton of guides to piano literature and they are all excellent. This guide to transcriptons, arrangements, and paraphrases has saved me hours of floundering through the music libary at the local university, looking for rarely-played music.
Along with music publisher information, Hinson provides concise descriptions of each entry and also rates how difficult each piece is.
As reference tools, Hinson's guides beat the pants off of hours of internet surfing. I highly recommend Hinson to all pianists who have ever torn their hair out trying to locate obscure music.