This book provides an excellent understanding of the qualitative physics of MRI. While other books tend to have a more practical approach and skimp on the physics, or even provide wrong or distorted information about the physics, this book systematically and exhaustively explains each topic in a down-to-earth, understandable way. Some basics physics are written as footnotes. There are a few chapters which I was unable to understand without looking at other sources (hence 4 stars instead of 5), but most chapters are easily understood.
This book should probably be used with online MRI physics animations.