Still reading, but it's already clear that this is The Book, not only on game design and not only for game designers, but basically required reading for everyone who aspires to have a holistic/systemic view of the design, development, teamwork, etc. processes.

Outstanding amount of references, breathtaking scope, which makes you think through a lot of aspects that otherwise look either too obvious, or unimportant, or get ignored in some other way.

Jesse Schnell puts all of these bricks in context and builds the big picture for you.
Also, the style and the language is impressively good, even more impressive if remember this is supposed to be just a reference!

https://www.goodreads.com/book/show/3396933-the-art-of-game-design