Prominence, I noticed from your other posts that you seem to be trying to choose btwn Psych, PM&R, and Anesthesia. That's fine - but IF you decide on PM&R, you may want to get at least one PM&R LOR. (I think you said you had a psych, anesthesia, and surgery LOR) I think all three are great fields - all have good lifestyles at least...
what you study would be dependent on what elective in PM&R you do. There are SCI, TBI, Stroke, post-Ortho, P&O, Wheelchair, EMG, Pain mangement, Sports Medicine, etc.... to name a few. Just read that section in a major PM&R text - like Braddom or DeLisa. If you want a more concise reference, I've heard that the Pearl series, the Secrets series, pocketpedia, and Cuccurullo's review book are all good. I wouldn't buy them as a student though - unless you have lots of $$. Most medical libraries and most PM&R dept libraries have these books. There are also references on the web - I used eMedicine's PM&R section as well as uptodate and Pubmed.
In terms of "warning", just make sure your interpersonal skills are good. Don't piss anyone off. Being a gunner does not pay off in PM&R. PM&R is very team oriented and pissing a nurse, a PT, a resident, a patient, and just about anyone in the rehab hospital can come back to haunt you. Unlike some specialities, the attending is not the only person to impress. Being arrogant, annoying, or too eager can all backfire. You could have all of Braddom and DeLisa memorized and have aced the boards, made AOA, and have several publications - BUT if you can't get along with others, they won't want you.
Also, even if you are trying to choose btwn PM&R and a specialty, or if PM&R is a back up plan (ortho is your first choice), do NOT share this with ppl at the program you may eventually want to get into. I have seen this happen to some of the nicest students. No one wants to know that you are trying to choose btwn Psych and PM&R. Ppl are not going to sell PM&R to you. You have to make that decision yourself. It's OK to ask why PM&R is good - but don't ask why PM&R is better than another specialty - unless you are asking someone you truly trust.
Make an appointment with the PD or dept chair during your elective so you can ask questions, get advice, and more importantly, make yourself memorable. Don't say or do anything during an elective what you wouldn't do during an interview. A PD once told me that she had a Harvard med student in her office and that student asked her how many IMGs and DOs were in her program because he doesn't work well with them. She didn't rank him and he didn't match in PM&R.
Anyways, hope this helps. I think the bottom line is to be nice, personable, and interested. If there are optional activities, do them. Stay a few extra hours. You can slack off after you match
just kidding. Seriously, think of the rotation as a month long interview and you'll do well. Don't let your guard down too much but don't be fake either.