It can be a core rule book, an adventure, a setting guide, a splat book, or even a tie in novel or comic book. What book directly related to TTRPGs is your favorite of all time.
You can use whatever definition of favorite you want, and of course your criteria are your own, but let us know the details!
I am going to pick Aaron Allston's Strike Force. That book was not just fun to read and about/a supplement for a game I love (Champions) it literally opened my mind about what a RPG campaign could be. I love that book so much I refused to buy the update because I didn't want to mar my memories or experience with a bunch of after-the-fact essays and such. (One day I will probably pick it up, tho.)
You can use whatever definition of favorite you want, and of course your criteria are your own, but let us know the details!
I am going to pick Aaron Allston's Strike Force. That book was not just fun to read and about/a supplement for a game I love (Champions) it literally opened my mind about what a RPG campaign could be. I love that book so much I refused to buy the update because I didn't want to mar my memories or experience with a bunch of after-the-fact essays and such. (One day I will probably pick it up, tho.)