Rulebooks for Reading

Over on Gnome Stew, DNAPhil recently broached the subject of reading game rulebooks for ideas on plots, mechanics, and even just for recreation.
I do all three of these myself!
I find that rulebooks from one system can provide inspirations for other games that I run. As a long time Amber GM, a game that spans a multiverse, nearly any setting from any rulebook can be used as inspiration for a shadow or world. Much of the Dreamlands in my world have been inspired by a host of supplements and ideas. The Yithonghu, enemies from the dream world, were originally inspired by a GURPS Cabal creature. A port city in Weirmonken, Turku, was inspired partially by an Iron Kingdoms Port city.
Even beyond Amber, I borrow from other games. For example, The ruins of Serathis, from Monte Cook’s Arcana Evolved world have shown up as a site in my face to face Exalted session. The magisterium, an ancient tower that was appearing only once every full moon, came from a session of In a Wicked Age that I played with the Indiegamers where that came up in the Oracles.
I fully expect to do more of this in the future. Beyond that, though, even if I never actually get to use or run them, I like to read RPG books for pleasure. Sometimes small things, ideas that I don’t even explicitly remember, come from pleasurably reading a nice and detailed RPG supplement.