There are a few good threads where books are discussed that you might find helpful. "PM&R books" should bring up a couple good threads.
PM&R Pocketpedia is the pocket book to have (I think there's a near-unanamous agreement on this). Personally, I used Grant Cooper's Essential Physical Medicine & Rehabilitation for more in-depth reading. Others seemed to really like PM&R Secrets, but I didn't find it useful--I just didn't like the format. Either one should be a great general synopsis of PM&R, so I'd just go with whatever format you prefer.
If you're doing a sub-specialty PM&R rotation (SCI, TBI, MSK, sports, etc.), then you could probably get a book more specific to one of those, but otherwise the Pocketpedia and one of the other two should be more than enough. I'd add an anatomy text as well, but most people already have those and hold onto them form first year.