I'm glad I sprung for the hardcopy of this for two reasons: one, I like to mark up my nonfiction, and two, its formatting! The left-hand page in every two-page spread is text; the right-hand page has an illustration related to the material on the left-hand page. While the illustrations are not technically the most accomplished, they are generally extremely effective communicative cartoons or diagrams.
This book comes with a ton of blurbs, and Cory Doctorow's--"Does for games what Understanding Comics [by Scott McCloud] did for sequential art"--pretty much sums up how I feel. I've read other game design books that were insightful, or thorough, but the Koster is accessible and very interesting in its approach to what makes games games, and how to make them fun (in the instances where that's a thing--cf. Brenda Romero's Train).
One of Koster's arguments is that "with games, learning is the drug" (40)--a game that interests us is one that strikes the necessary balance of not too easy (Tic-Tac-Toe, for most adults) and not too hard (multiple failure modes possible, depending on the individual--witness me and chess or go ). He suggests that games (and play, which is common in a lot of young animals!) are an artifact of how we try to learn survival skills, and moves forward into making suggestions as to how to move the form forward into values/skills more suitable for the modern era than "kill things" or "jump over things" or "search for all the things."
 Joe gave up on teaching me go when I told him I have severe difficulty with visual patterns. In fact, I am starting to wonder if aphantasia just screws me over for this kind of game in general. :p
There's also a particularly interesting chapter on ethics and entertainment where he discusses the difference between the game system and the flavor/dressing:
The bare mechanics of a game may indeed carry semantic freighting, but odds are that it will be fairly abstract. A game about aiming is a game about aiming, and there's no getting around that. It's hard to conceive of a game about aiming that isn't about shooting, but it has been done--there are several gmaes where instead of shooting bullets with a gun, you are instead shooting pictures with a camera. (170)
The bare mechanics of the game do not determine its meaning. Let's try a thought experiment. Let's picture a mass murder game wherein there is a gas chamber shaped like a well. You the player are dropping innocent victims down into the gas chamber, and they come in all shapes and sizes. There are old ones and young ones, fat ones and tall ones. As they fall to the bottom, they grab onto each other and try to form human pyramids to get to the top of the well. Should they manage to get out, the game is over and you die. But if you pack them in tightly enough, the ones on the bottom succumb to the gas and die.
I do not want to play this game. Do you? Yet it is Tetris. (172)
In general, Koster has a background in game design AND writing AND music, and he draws on all three in his analysis of games, as well as other disciplines (e.g. psychology). It makes the book a scintillating read. I can't believe I waited so long to read this--but it was exactly what I wanted to read last week, so hey. Highly recommended.