Book: The Art of Game Design by Jesse Schell
This is a shockingly comprehensive, and at the same time very easy to read, introduction to game design. The book touches on aspects ranging from game mechanics, aesthetics and technology considerations, to teamwork advice and design documents.
Overall, the book gave me a lot of respect for the complexity and size of the field. As a game designer, you have to draw from a big number of disciplines such as anthropology, psychology, sociology, architecture, probability theory or graphic design. You have to think about what needs of the player your game satisfies (think Maslow's hierarchy, but not only), how to maintain the difficulty curve so that the player is in the "channel of flow" (not too easy, not too hard), how to balance your game with respect to different axes, what theme the game will have and how to use every aspect of design to reinforce that theme, and so on and so forth. There is also some advice on brainstorming and working with a team which can be easily applied outside of the field of game development.
All these ideas are neatly categorized in the book's chapters, and in each part I found some interesting insight. To give one example, there's a concept of game venue as something that defines the type of play experience. The author mentions venues like the hearth where people gather together (modern hearth being, well, the TV with a console), your personal workbench where you concentrate on things (desktop PCs would fill this niche, with PC games being more "serious" and less casual), the reading nook where you sit comfortably with a book (or a tablet), a table for board games, public spaces like arena for competitive games, and so on.
Another example is the well-known notion of "emergent gameplay" that somehow always felt like a magic to me – my understanding of it was "create a sufficiently complex game and it will magically become more interesting because of what the players come up with". The book breaks it down nicely: a game can have basic actions which are basically rules of the game (for instance, move a piece or capture a piece) and strategic actions which are implied by these (for instance, move a piece to protect another one, sacrifice or exchange pieces, force an opponent to do something…) It's generally good to have a small number of basic actions from which many strategic actions can emerge. A good way to achieve that is to make each of this actions meaningful, e.g. have far-reaching consequences instead of just local ones.
That's just a small sample of things I took away from The Art of Game Design. I borrowed the book from a friend but I think I will be buying my own copy, since I definitely plan to return to it in the future.