If you only buy one book about game design this should be it. Covers pretty much everything you need to know when starting out. Also contains really interesting interviews with different game designers.
The book focuses on two things. One is prompting you as potential game designer to become very analytical about games that exist already, breaking down what works and what doesn't and why. The second is an iterative approach to game design, starting with a paper version, playtesting early and often.
It contains everything you need to start designing your own games, learning how to balance and polish them, plus interesting interviews with people working professionally in the field. If your focus is actually making original games rather than studying them academically or learning more about them as a player, this is a very useful book.
I bought this for my son as a present, as he wants to design his own board games. He is absolutely thrilled with it. It contains a mine of information, and the case studies are really useful. It tells you all you need to know, including testing, marketing, storyboarding, etc. Highly recommended.