This post is in honor of the books that I bought just because I can't count on the library's copy surviving dog-bite, immersion in bathtubs, and other occupational hazards of a library book.
Castles in the Air is the book I trot out every time we have any kind of "staff picks" display. In 1994 Judy Corbett and her then-boyfriend-now-husband decided to buy a ruined Tudor castle in Wales called Gwydir and restored it while operating it as a bed and breakfast. Corbett tells stories that make you want to invite her over to dinner and listen to her for hours (like the really creepy one about one of the resident ghosts, or the funny story involving Prince Charles).
Seabiscuit by Laura Hillenbrand. Yes, there was a movie, but it skipped all the fascinating details about horse racing and the Great Depression, and the lives of Seabiscuit's jockey and owner. The rivalry between War Admiral, Seabiscuit and their owners will suck you in.
Fiction I can't part with includes the previously blogged The Amazing Adventures of Kavalier and Clay by Michael Chabon, How I Live Now by Meg Rosoff (because you always need a book about a teenager and her cousins on their own in rural England when World War III breaks out) and Garth Nix's Abhorsen trilogy, in which an untrained teenaged necromancer goes into the land of the dead to retrieve her father.
Do you have any books you must own even if you never get around to re-reading them?