Erin McKean, dictionary editor and author of Weird and Wonderful Words, recommends these books:

Word Origins... and How We Know Them: Etymology for Everyone by Anatoly Liberman

The Warden of English: The Life of H.W. Fowler by Jenny McMorris

Word Myths: Debunking Linguistic Urban Legends by David Wilton

Lapsing Into a Comma: A Curmudgeon's Guide to the Many Things That Can Go Wrong in Print--and How to Avoid Them by Bill Walsh

Slayer Slang: A Buffy the Vampire Slayer Lexicon by Michael Adams ("Just in case any of you thought I was joking, the essay on ephemeral language is worth reading even if you've never seen an episode of Buffy," says Erin)