Very good theory books on game design?

I like "Challenges for Game Designers" by Ian Schreiber as a solid, broad-base book on design. "Rules of Play" is more thorough/in depth, but it's a much harder book to slog through.

Here's my more thorough list (though it doesn't include a couple of books that were published this year):
http://www.truekidgames.com/DesignEducation.html

I've also read The Art of Game Design my Jesse Schell and I totally recommend it. It's a very motivating book that encourages the reader to look at their designs from many perspectives.

I have read rules of play and it is a really good book on game design theory. Some people complain about it, because of it is pure theory without any practice. But that is what the book is about and in my opinion you should read it before you start learning game design practice.

Rules of play gives you an idea what game design is about and what makes up good game design and how to do it properly. It also introduces all those theories game design borrows from other fields of research. That might sound a little scientific and difficult to understand, but there are a lot of examples of well-known games used to explain certain game design concepts.

It also provides an introduction into the workflow of game design and gives you an idea how you should solve game design problems. One thing I have learnt from Rules of Play for example, was that you should fix problems by introducing a new feature rather than adding another rule that just works around the problem. If possible you should also target multiple design problems at once, so the change you do to fix them integrates well into a larger part of the game and contributes more meaning to the play. It also explains why play testing your game regularly and as early as possible is important. That's because of games and even more the players and the fun they experience while playing your game are way to complex to always respect when designing your game, that you should rather observe what affects each change you do to your game has on its play experience.

That were only two, but the most important things I have learnt by reading Rules of Play. I recommend to start with that book, if you are interested in game design or designing games your own and looking for new ways to solve design problem and make your games more meaningful and thereby more fun to play.