There are many books for GIMP. I randomly choose this book based on the community feedback and I have not regret it. I could have learned the basic features and usage of software form this book. It has sevred my perpose very well.
I took lots of learnings from this book. Command promt is an essential tool for linux users. Personally, I like Mark Sobell's book. It is easy to unerstand them and makes light introduction to heavy topics.