This is being covered in one of the other MCW threads, but I'll put my $.02 in because I'm bored waiting for a cath to start.
Path: Robbins Path (yeah, the big one) is probably worth buying for clarification purposes and the histopath pictures, which tend to find their way onto exams. Goljan's Rapid Review Path is fantastic (and cheap!) and I've heard rumors that the path course is reorganizing somewhat to fit this book more. The Robbins Review question book is also a great resource that I felt helped me a lot on exams and Step 1.
Micro/Immuno: I really like Micro Made Ridiculously Simple, although it probably isn't essential since the course notes are pretty comprehensive. Ashers really hated MMRS and recommends longer, more involved texts. Please keep in mind that she is a bigger textbook reader than your average student. She and I are like the Jedi and Sith on this one, but we're able to remain homies. For immuno I really liked the Basic Immuno book they recommend. I had no prior exposure to immuno and I felt like this helped me catch up with those that took it in undergrad. The course notes and figures for that block also come mostly from that book.
Psych/Ethics/Health Policy/CER: Don't bother buying any books. Yes, even Bates will not be useful. If you must insist on a Bates to complete your med reference library, at least get a used copy or older edition. I opened mine twice all year.
Pharm: No book necessary, the notes are very comprehensive (please read: long). I picked up Trevor & Katzung's Pharm Review, which was probably somewhat helpful, but likely not worth the cash overall. Be sure to read along in First Aid in this class, because they tend to not teach to the boards on this one and you really should learn some of the heavily Step 1 tested side effects for each drug as they are covered in class. Will make pharm on Step 1 a breeze.
There you go. Be sure to peruse First Aid throughout the year to get a handle on what topics are truly high yield for the boards and which are probably just some MCW prof's pet subject.