A librarian attempts to uncover a patron's hidden past, revealing a story of lost love and the discovery of DNA. Thematically, this is exactly what you'd get if Douglas Hofstadter and Neil Stephenson cowrote a novel (music! science! sysadmins!), but that makes it sound like much more of a mishmash than it really is: it's beautifully constructed and a very pleasant read. Also, I'm not going to complain about any book that gives me an excuse to listen to Glenn Gould playing Bach.
Thoroughly recommended; going right up there with GEB and Life A User's Manual on the favourites list.