For micro, there's almost no book that can rival with Microbiology Made Ridiculous Simple (MMRS). I've read 80% of it and will finish in the next few days. Don't be scared by its size. You will be AMAZED by how fast you can read through it, and most importantly, it's sometimes even fun to read it. Can you image reading a board review book and be ENTERTAINED? Well, this book made it possible. And its coverage of microbiology for Step 1 is very complete.
The downside of MMRS is that it has no practice questions. Also, the tables after each chapter are mostly reiteration of what's in the text. I just skip it because I don't learn a lot from tables.
As for immunology, it's pretty much been concluded that a good Immunology book does not exist. The thing with immuno is that it's very hard to study immunology as an isolated subject. It's always integrated into other topics, like lung disease (asthma), autoimmune diseases, and infection. The immunology section in FA is totally worth pure memorization. If you can take nothing home, make sure you know the hypersensitivities as second nature.