Discworld 3: Equal Rites by Terry Pratchett

Terry Pratchett's profoundly irreverent, bestselling novels have garnered him a revered position in the halls of parody next to the likes of Mark Twain, Kurt Vonnegut, Douglas Adams, and Carl Hiaasen.

In Equal Rites, a dying wizard tries to pass on his powers to an eighth son of an eighth son, who is just at that moment being born. The fact that the son is actually a daughter is discovered just a little too late.