Ahhh, London - I love London! It's got such an incredible pulse to it and the mix of old and new can really blow your mind. If you're going there, definitely check out Westminster Abbey and TALK to the people that work there - they are very, very knowledgeable and can fill you in on all sorts of cool stuff you'd otherwise miss. For more London info, check out this thread:
If you were going to London, England
I'll throw in another vote for Paris - it's absolutely spectacular. I've been there twice and never had any problems getting around or encountering rude Parisians, and I only speak "travel French". Go with an open mind, realize you're not in America, and if someone
is rude to you, remember that not every single American you've ever met has been super-friendly. As for Paris being dirty, I really didn't think it was any dirtier than most American cities. Chicago isn't always a pristine place, either.
As for guidebooks, my absolute hands-down favorite is Rick Steves. I've checked out Let's Go, Lonely Planet, and Frommers and while they're all not bad, none of them have the spot-on recommendations (and humor!) of Rick Steves.
HAVE FUN no matter where you go! Europe is awesome, and now is a great time to travel!