This book is a must for everyone who wants to understand how CG works and to create applications including textures and terrain generators, together with shaders for realtime and other.
I put 4 stars, even if I consider it a 5 stars, only because I had to read some chapters twice and together with other online materials just to understand better, but it's just me, I mean, I'm not a genius in math, but they use a very common language, especially the chapters of Dr. Musgrave.
So if you know the matters, better for you, otherwise put a lot of attention.
The book is very inspiring: I'm applying its theory with SideFX Houdini in my free time and I'm learning new things day after day.