I agree w/ PM&R Secrets... it was very helpful during my (5😱) PM&R elective rotations last year. It's thick, but you certainly don't have to read all of the chapters. The intro chapters on anatomy, physical exam, etc are great and you can just read the few other chapters on whatever area of PM&R that you are rotating in.
Choi's Pocketpedia is also a popular and solid choice, but may be too concise to learn from as primary source for these rotations. I always have it in my pocket as a quick reference, but if I had to pick one as a student I'd go w/ Secrets.
Regardless of what area of PM&R you are rotating in.... KNOW YOUR NEUROMUSCULOSKELETAL ANATOMY COLD!! The book probably doesn't matter, but I can't stress enough how this was the one area that I was almost uniformly expected to know.... not to mention that it's extremely important in all areas of PM&R.
If you are doing a sports/musculoskeletal rotation, make an attempt to at least know how to perform manual muscle testing and common physical exam maneuvers. Residents and Attendings should help you refine your technique, but you shouldn't be starting from scratch on the rotation. Hoppenfeld's "Physical Exam of the Spine and Extremities" is a fairly basic and classic book. I'm a big fan of Malanga and Nadlers "Musculoskeletal Physical Examination". The DVD that comes with the book is great too.
In terms of places to rotate, I don't know much about the programs in Baltimore/DC, but I did rotate at Temple and Jeff. Both of those programs fit the 3 criteria that you mentioned and were fairly easy to set up rotations with.