The three books I've read and then gone back to over the years are:
- Brenda Romero (credited on the book as Brenda Braithwaite) and Ian Schreiber's Challenges for Game Designers[1], which uses a series of design exercises to illustrate concepts like balancing and mechanical loops, and at least from a cursory glance at the Riot curriculum is very similar in structure to it. The exercises still help me bootstrap my understanding of mechanics that I don't always employ in designs, and still occasionally inspire new ideas just by going through one almost like a karate kata.
- Raph Koster's Theory of Fun for Game Design[2], which is more introspective about the nature of games rather than a pragmatic how-to guide. Koster's views are often contentious, particularly on defining what a game is or can be, but I usually go back to this book when I want to step back and remind myself what kinds of audiences might be in my blind spot for a mechanic or concept — I might think something is fun because I think it's fun, without interrogating why, and even if I don't agree with Koster on how he goes about defining it the book does a good job of demonstrating how one builds a definition in the first place.
- Jesse Schell's Art of Game Design: A Book of Lenses[3], which just got a 3rd edition last year that I haven't had a chance to read yet. It's a framework for interrogating a design — and I say "interrogating" literally, as the lenses are a set of questions to ask of your design — that touches on mechanics but also how viable an idea is to make into a game in the first place. Walking an idea through the lenses serves like a second pair of "eyes" when I don't have another person handy to bounce that idea off of in depth. There's a card deck version[3] of just the lenses that are handier to have on the desk.
It's likely that there are newer books out there covering similar ground;[4] these are the ones that were around when I came up through my first game design experiences around 2010-2012.[5] These are all high level enough to be pretty general works on the nature and purpose of all types of games, even if all of them have digital games at or near front-of-mind, without being so focused on theory and philosophy that they don't give you actionable things to apply in a design.
(Also, I've found that all three of those names are polarizing, often for very different reasons. I find the works valuable regardless; books don't tend to yell at me in a Discord chat or on social media when I read them, which is a nice change of pace.)
[1] https://www.amazon.com/Challenges-Game-Designers-Brenda-Brat...
[2] https://www.powells.com/book/theory-of-fun-for-game-design-2...
[3] https://www.schellgames.com/art-of-game-design/ — and of course there's a booster pack available, because game designers are insatiable post-publication tinkerers
[4] Procedural Storytelling in Game Design, co-edited by Tarn Adams of Dwarf Fortress and Tanya X. Short of a bunch of procedural indie games and formerly Funcom, is exciting in concept, and Darius Kazemi is always fun to read on the subject: https://www.amazon.com/Procedural-Storytelling-Design-Tanya-...
Building Blocks of Tabletop Game Design, by Geoffrey Engelstein and Issac Shalev, looks like it ports the cookbook concept from software development to tabletop games, which feels right up my alley: https://www.amazon.com/Building-Blocks-Tabletop-Game-Design/...
(It seems notable that both of those are by CRC Press, which I associate more with academic textbooks, but neither really fits the usual bill for them.)
Calling it "newer" might be a misnomer, but I'm desperate to get around to the illustrated The Making Prince of Persia: Journals and wish there were more books with the perspective of a journal during development — watching along as opportunities open and close, instead of as a post-mortem that's colored by the end product and invariably focused on what went wrong/how to avoid it. Also it just looks gorgeous: https://www.amazon.com/Making-Prince-Persia-Journals-1985-19...
[5] Steve Swink's Game Feel has been on my list of to-reads on the subject forever, but I've never gotten around to it: https://www.powells.com/book/game-feel-9780123743282
I bounced off Katie Salen/Eric Zimmerman's Rules of Play, but it's lauded enough that it's hard to omit it. If you like Koster's ideas around orthography in game design and want to see a predecessor that takes it further and issues prescriptive rules, or if you have more of an interest in design criticism than creation and want an intro to a critical vocabulary, it might appeal more: https://www.powells.com/book/rules-of-play-game-design-funda...
Tracy Fullerton's Game Design Workshop is a great read, and even more well suited to a classroom environment than Challenges for Game Designers, but it's _too_ classroom-y for my tastes, focused more on getting from point A to a very specific and more directed point B, instead of laying out a more open-ended task where the boundaries can be a creative aid. I've never felt compelled to return to it as a result, but there are folks who swear by it, so it's still worth mentioning: https://www.amazon.com/Game-Design-Workshop-Playcentric-Inno...