Previous such incidents left me with such treasures as my annotated copy of the fire code. This time around it was an encyclopedia of the rules to 150 major world sports (including jai alai, canoe sprint racing, bandy, and flat green bowls) and a 1966 surveying textbook. I'm especially happy about the latter; I've been looking for a good technical introduction to surveying for a while. Most modern books on the subject seem to omit much of the underlying math and physical principles, but this one predates all our modern fancy-schmancy electronical devices. GPS is for amateurs, and electronic sights are for cheaters; I want to know how Mason and Dixon did their job.
There's an alternate universe in which I'm a reference librarian, as opposed to the intellectual dilettante I am in this one. Every so often, when the dimensions line up, that alternate me crosses into this reality, hits the bookstores, and leaves me with books that are cool to have but not exactly useful.