Next Generation has an article up by Ernest Adams entitled 50 Books for Everyone in the Game Industry. I'm honoured that he has decided to include both my non-fiction books in the list - my game design book with my friend and colleage Richard Boon, and the game writing book I edited for the IGDA Game Writers' SIG (both plugged on the sidebar here). Now Ernest is not necessarily unbiased in this regard, since he contributed to both, but he is a highly ethical individual and wouldn't recommend these books if he didn't think they were worthwhile. (He doesn't plug his own books in the list, you'll notice).
The list also contains all manner of books I would recommend myself, including Sheri Graner Ray's Gender Inclusive Game Design (short but eye-opening), Caillois' Les Jeux est les Hommes (my favourite game design book that isn't about game design at all), and Campbell's The Hero with a Thousand Faces (I prefer his later work, but his writing on mythology is unparalleled).
Since Ernest couldn't plug his own books, I will do it for him. Firstly, for people who are interested in a career in games development, there is Break Into the Games Industry: How to Get a Job Making Video Games, which will suit anyone with absolutely no idea how to begin a career in games. Secondly, there is Andrew Rollings and Ernest Adams on Game Design, which takes a broad genre-based look at game design, ideal for anyone starting out in the field, and anyone looking for a coherent examination of the game design process. Lastly, there is a new expanded version of this book, retitled Fundamentals of Game Design, which I haven't seen yet but which I understand expands the content significantly to create a text book suitable for a course on game design. I look forward to seeing it soon.
Don't forget to check out Ernest's book recommendations at the Next Generation website!