One of the things that can make or break your vacation is a good guidebook. This is especially true if you're traveling overseas. My husband and I have purchased mountains of guidebooks (some to countries we haven't even visited yet) and I think we can safely say, these guides are our most favoritest:
Rick Steves specializes in Europe "through the back door". You can catch him on PBS touring all the great cities of Europe. He's also got a very comprehensive website, filled with great information for anyone planning a Euro tour. We brought his guidebook to Italy with us on our most recent trip, and it proved to be insanely helpful. In addition to the guidebook, we also downloaded a few of his podcasts onto our iPods.
This was awesome for several reasons:
1) they were free, 2) you look much cooler listening to an iPod whilst traipsing through centuries old ruins (rather than those blocky audio guides that cost more than a mortgage to rent), and 3) you learn a lot outside of "this place is old and something important happened here."
I'm also a fan of the Lonely Planet guidebooks, available for just about anywhere you care to travel - international or stateside. A fair warning - depending on where you're traveling, the books can take the form of a bible/concrete block/small village in size. The New Zealand book weighs roughly 8 pounds. But if you're looking for a pizza place in SmallTownWhereTheHeckAreWe, New Zealand, this book will tell you where to find it. These books tend to have a bit of a bias toward anything chain related (hotels, restaurants or otherwise), but are overall very useful.
Consider investing in a decent guidebook before you go anywhere that involves major sightseeing. And if you don't, good luck finding decent pizza in New Zealand.