Oh, no! Your house is suddenly going up in flames! You have time to save all your animate loved ones (children, pets, housemates, etc.) a handful of vital possessions, and ONE BOOK from your beloved collection. (Just one!) Which book will you choose? (Decide quickly!)
There are some good answers (and agonizing!) in the comments to the entry above. But the decision was surprisingly easy for me: the German-language Bible that my grandfather gave me. Not at all practical, but it's irreplaceable in at least two ways.
So how about you?