I just started reading "The Adventure of English" by Melvyn Bragg, which goes through the history and evolution of English from Old English through the current day. As I'm reading about the ways that each successive wave of invaders changed the language, it seems like there must be a huge amount of history behind Spanish and all of the regional languages, given the history of Spain itself.

Does anyone know of a good book that gives the history of the Spanish language (or even Catalan)?

For Spanish, try "History of the Spanish Language" by Ralph Penny

Also, for other good books on the history and quirks of English, I HIGHLY recommend:
"Mother Tongue" and "Made in America" both by Bill Bryson, the author alive today!

These books will not disappoint