My recommendations:
First AID USMLE Step 1. It's not a required book, but buy it. It's in my opinion the most important single book for the first 2 years.
Get a Path review book (BRS Path or Goljan) or both...i recently got Goljan and while I like it a lot, I think BRS path may be better for exams. It does a better job explaining things. You should also probably get a Robbins although I haven't really used it as much as my review books to be honest.
Get neuroanatomy through clinical cases for neuro. I loved that book.
Get Moore & Dalley and One of the picture anatomy texts that show actual cadavers.
Whatever you do DONT BUY BATES.
I would also get a Harrison's. You can access the text online, but I hate reading off a computer. Harrison's pretty much has everything in it.
Microbiology Made ridiculously simple- the only resource I would use for microbio. I first got it this year and it's really helped me in microbio. I wish I had it last year.
OMT by Savarese- the best book for OMM. Great for the Boards as well.
Immuno by Parham...I liked it, not necessarily a must have.
Anyway, in my opinion, that's all you'll need. Most of the required texts are terrible books and 99% of lecturers take most of their material from their powerpoints. The books you need should really be so you can focus on studying for the boards, not the exams. To do well on exams focus mostly on the powerpoints and on First Aid.
Anyway, that's my opinion, I hope this helps.