Guide Book Sites
AAA.com - Region specific and pretty good.
BlueGuides.com - History and art oriented.
DK.com - A lot of pictures, I've used several of these.
Dummies.com - Actually OK, but embarrassing to carry.
Fodors.com - A lot to choose from and comprehensive.
Frommers.com - Detailed and a wide selection.
Insiders.com - Travel guides for the USA.
InsightGuides.com - Almost the same as DK, but not quite as good.
LonelyPlanet.com - Exceptional and popular.
LetsGo.com - Great for the budget oriented traveler.
Moon.com - Excellent and comprehensive guidebooks.
RoughGuides.com - Very budget oriented.
RickSteves.com - Great European guides.
ViaMichelin.com - The green guides are good.

Online Stores To Pick Up Guide Books
Amazon.com - Use it all the time, new and used.
BarnesAndNoble.com - Another great place for books.
eBay.com - What can't you find on eBay?
GlobeCorner.com - Travel books, maps, and guides.
Walmart.com - Walmart has a lot of things including guide books.