Brathwaite and Schreiber's "Challenges" is a great introductory work for those starting to learn about game design, a helpful training tool for the intermediate designer, and a good candidate for a textbook for educators teaching design.
For the beginner, the book is clearly laid and approaches design from the very basics, giving a reader who possesses a zero knowledge base a solid grasp of the core concepts and processes of game design. The first two parts of the book cover individual topics of design chapter by chapter, walking the new designer through different elements of design, from incorporating elements of chance to playtestesting for balance. Even more importantly, the book takes a hands-on approach to design and requests that the reader try out their new-found knowledge by completing "Challenges" - short exercises found at the end of each of these chapters which ask the reader to build a game using a specific core concept. Each of these challenges require that the reader build a non-digital game, so even readers with no programming ability can quickly jump in and try out their new skills. Overall the book is written in a friendly, informative and professional manner, and should be on any beginner designer's must read list.
For the intermediate designer, the book has even more to offer. The afore mentioned Challenges are great tests for thinking about design in new and different ways, with the "Iron Designer" challenges offering an even more complex task. The latter half of the book discusses design from a more theoretical and professional approach, with topics ranging from games as art to working with an IP. The beginner may also find many of these discussions useful and informative, but some of the topics are definitely aimed towards those already in the professional industry.
Finally, the book even manages to cater towards the education and serious market, with topics dedicated to games as a learning, training and socializing tool. Teachers may find the book useful as a textbook because of its concise writing yet exhaustive depth and breadth, and many of the Challenges are well-suited for student assignments exactly as they are written. There is easily enough material for at least a semester's worth of study, and the book's low cost for content makes it more affordable for students and institutions than other volumes on the market.
This is possibly the best intro/intermediate design book I've read to date and can't recommend it highly enough to anyone interested at all in game design.