One of my professors told me that as medical students we learn more (and unfortunately forget more!) in the four years than most people learn in a lifetime! Having gone through it I would say he is absolutely correct! The vast amounts of knowledge in the medical sciences makes picking the proper books to study from even more important. In general I recommend avoiding books that are likely to change frequently over time and stick with the ones that you'll go back to repeatedly. Below are a few books I found particularly useful during my years as a med student. They are organized by year with a brief explanation of why I chose it.
(1) Robbins & Cotran Pathologic Basis of Disease - Although expensive this book has every single fact about pathology that you'll need to know. It covers every organ system and disease in excruciating detail. Not great for primary reading, but if you ever need to look up anything it is in here!
(2) Pathophysiology of Heart Disease: A Collaborative Project of Medical Students and Faculty - This books is the collaborative effort of Harvard faculty and medical students. It is extremely well written and easy to understand. I referenced this book numerous times not only during 1st and 2nd years but into the clinical years as well.
(3) Clinical Microbiology Made Ridiculously Simple. Great for learning the basics of microbiology. It has all the important bugs and anti-microbials that you'll need to memorize. Written using funny language and pictures that helps the info stick. If you need more than this for medical school then you're probably headed towards an infectious disease fellowship.
(4) First Aid for the USMLE Step 1 - The "bible" for USMLE step 1 studying... Need I say more?
(5) Goodman and Gilman's The Pharmacological Basis of Therapeutics - Again another pricey book, but one that will become part of your medical library throughout your training. It has everything you need to know about pharmacology and medications.
(6) Duus' Topical Diagnosis in Neurology: Anatomy, Physiology, Signs, Symptoms - Excellent illustration of neurology and neuroanatomy. This is really a basic sciences course book, but I like it because it has a very heavy clinical bent, which prepares you well for the clerkships.
(7) Renal and Electrolyte Disorders - Great book, highly recommended for understanding those darn pesky kidneys! And it's relatively cheap!
(8) Clinically Oriented Anatomy - This book is essential, especially for all the possible surgeons in the room. I used this book along with the atlas (next on the list) to get a good grip on anatomy.
(9) Color Atlas of Anatomy: A Photographic Study of the Human Body - I tried using Netter's (which is a good atlas), however I always found looking at drawings to be too unrealistic and idealized. This book is excellent because it has hundreds of photos of actual cadaveric dissections. It is still in my library today and I reference if frequently. Caution - avoid opening this book on subways, buses, etc as the pictures are quite graphic!
(10) Pulmonology Review - Good review of pulmonary physiology. This book is also helpful when you get onto the wards as well.Pocket Medicine: The Massachusetts General Hospital Handbook of Internal Medicine - This is one of the most used handbooks on the wards by both medical students and residents. I liked it because it is a binder that you can add your own notes to if you'd like.
(2) Bates' Guide to Physical Examination and History Taking - Everything you'd need to know about the physical exam. Again a little on the pricey side, but one of the books that will stay in your medical library regardless of what field you go into.
(3) MKSAP For Students 4 - This is a book of questions that helps prepare you well for the shelf exams. I like this book because it gives you the references of papers that you can look up and read on pubmed.
(4) MKSAP 15: Medical Knowledge Self-Assessment Program, Parts A & B - This set includes several books with questions about various general and subspecialty areas within medicine. The informative sections are written based on clinical evidence from published papers (which I loved!). The set is pricey, so you might be able to pick them up at the library, or pick up individual topic books (ie: cardiovascular, pulmonary, etc.) depending on what your interests are.
(5) The Only EKG Book You'll Ever Need (Thaler) - The name of the book says it all...
(6) Psychiatry 2008 (Current Clinical Strategies) - This was the only book I used for psychiatry in addition to the lecture notes, etc. given by professors. I like this book because it fits easily in your white coat and is a quick reference to most of the psychiatric diseases you'll need to know. If you don't plan on going into psychiatry this is likely all you'll ever need... Plus its relatively cheap!
That's my list and I'm sticking to it! Although there are tons of other great books out there I felt that these books were the best for my personal learning style. If you have any books that you absolutely love and couldn't live without let me know and I'd be more than happy to add them to the list!