It it very comprehensive - it covers many different aspects in depth, from the basic mathematics, representing shapes, techniques for accelerating the rendering process and camera models to sophisticated lighting models, sampling theory, textures, volume scattering and much more. It simply contains a wealth of information on almost every aspect of ray tracing.
But it's not only excellent because of the content; also the style in which it is written is very clear, and also the print quality of the book is exceptional - it's printed on high-quality glossy paper which makes it a pleasure to read.
If you're interested in ray tracing, you will certainly not regret buying this book, and it will keep you busy for a long time.
Put simply, this is the best book i've bought on ray-tracing (and i've bought a few!). If you are looking for practical, then this is the most practical book you could buy. It does explain the maths theory, but with an emphasis on programming it. I totally recommend this book.