The following review books and resources are the ones I found most helpful throughout medical school and are kept up to date (last revision January 2020). Reading these can help you learn the topics most likely to be tested on the USMLE Step 1 exam.
If you are looking for USMLE Step 2 or 3 books you can find them here.

First Two Years of Medical School – The Basic Sciences and Step 1 Prep

General USMLE Step 1 Board Review:
Even if you are not preparing for Step 1 in the next few months it is a good idea to get a copy of First Aid for the USMLE Step 1 and read through it as you go through your classes in the first 2 years of medical school – that way, when the time comes to really prepare for the exam, everything will literally be a review (instead of trying to learn any new material).
Anatomy & Embryology:
This is primarily just memorization. You will need to memorize all kinds of esoteric facts to do well in the class that will not be tested on board exams, although many USMLE questions require that you know the basics. The best review books are Rapid Review Gross and Developmental Anatomy and BRS Gross Anatomy. I would pick one of those to study along with memorizing Netter’s Anatomy Flash Cards.
Biochemistry is not heavily tested on board exams but there will be some questions on Step 1. Questions involving the various biochemistry syndromes, such as specific enzyme deficiencies, are most common and are covered well in Rapid Review Biochemistry.
Cell Biology and Histology:
Rarely tested on the USMLE, the most common questions contain images that show a very characteristic histology specimen of a specific disease (which you will likely see again in pathology) or a type of tissue damage. If you choose to get a review book for this subject BRS Cell Biology and Histology does a pretty good job of covering the basics.
Behavioral Science:
There will be some behavioral science questions on the USMLE. If you have taken (and remember) any psychology and statistics classes in college that will be helpful, but for everything you don’t already know, BRS Behavioral Science is a good outline and guide.
Microbiology & Immunology:
Rapid Review Microbiology and Immunology is one of the best overviews of microbiology. The microbiology questions seen on Step 1 are often a combination of micro and pathology. Lippincott’s Microcards: Microbiology Flash Cards are great for helping with all the memorization necessary for this subject.
Although not tested extensively, neuroscience questions do appear on Step 1. USMLE Road Map Neuroscience is a good review of the basics.
The most heavily tested subject on the USMLE, by far. You need to focus on this as much as possible the first two years – don’t just memorize and forget – learn it so you know this material. It will be the most important subject for Step 1 as well as helping you better understand everything else in medical school and your career as a physician.
Rapid Review Pathology is written by Edward Goljan (the famous Step 1 review course guy), and is extremely good – I would definitely study this book.
Robbins and Cotran Review of Pathology has over a thousand USMLE vignette-style questions with thorough explanations and should be used in addition to Goljan’s Rapid Review.
Katzung & Trevor’s Pharmacology is a pretty comprehensive (long) review book, but can really help you as you understand some of the more important topics. Pharmacology by itself is not often tested on the USMLE Step 1, but it is frequently used in combination with other subjects like pathology and physiology, so you do need to know the basics.
USMLE Road Map Pharmacology does a good job of covering the basics – it is much shorter and is also written by Katzung. To help memorize the many things you will need to know in your pharmacology class, Lange Pharmacology Flash Cards are helpful.
Physiology is extremely important and is tested extensively on Step 1, only second to pathology. You will need to know both – what is normal (physiology) and what is abnormal (pathology) to do well on any USMLE exam. To learn normal physiology, BRS Physiology is an excellent outline and is very well written – the best of the BRS series.