The intro book we hand to beginning students is:
MRI The Basics (Hashemi)
http://www.amazon.com/MRI-Basics-Ra...bs_sr_1?ie=UTF8&s=books&qid=1196125864&sr=8-1
This book gives a good summary of MRI with a lot of pictures. It's not for the physics/mathematically oriented reader but is the best intro text, especially for clinicians or those who don't have a lot of time and want to get up to speed quickly..
The older but good text that we recommend for students now is:
Magnetic Resonance Imaging: Physical Principles and Sequence Design
http://www.amazon.com/Magnetic-Reso...=sr_1_1?ie=UTF8&s=books&qid=1196125924&sr=1-1
This is a very good textbook and heavy on the mathematics. It gives an excellent treatment of many introductory topics and if you can slog your way through this book you will understand MRI very well.
The book that everyone loves (with good reason) nowadays is:
Handbook of MRI Pulse Sequences
http://www.amazon.com/Handbook-Puls...=sr_1_1?ie=UTF8&s=books&qid=1196126042&sr=1-1
This book is great on so many levels. It is practical. It coves the vast majority of the things you will encounter in MR pulse sequences. It describes things both quantitatively so that you could almost do it yourself or qualitatively if you want a basic understanding without mathematics. It does talk about some fundamentals of MR but I'm not sure I would make this the primary book to learn on. Still, I would recommend this book to ANYONE who works in the field of MR, especially in a pulse sequence design or protocol optimization capacity. As a learning book, one of the ones recommended before this one will suffice depending on the depth with which you want to cover the basics.
There's still a very important question that someone will raise so I might as well raise it first. Why do you care about this as a medical student? If you are doing MR research projects like me, that's great and I hope this is of help to you. If you're just reading this for your interest in Radiology, you're wasting your time. If you really want to learn about the practice of Radiology, you should pick up Squire's or some clinically relevant intro. To read this book if you're interested in Radiology to me is almost like reading Albert's Cell Biology because you're interested in Internal Medicine.