Lonely Planet travel guides
Lonely Planet is very well known and respected in the travel guide book arena, they seem to be able to pack a huge amount of info into a handy sized book, and once you learn the format and abbreviations, it is a simple matter of finding pretty much anything you want about a place or country, including an extensive list of hostel and backpacker accommodation!
Rough GuideRough Guide also has an extensive collection of guide books, I have only seen a few examples in bookstores, and have not used them in the field, so I can't really comment on there performance, but am sure they fare well!
DK Eyewitness travel guides
DK Eyewitness travel guides are one of my favorites, they may have a little less info than the Lonely Planet guides, but they more than make up for it in the more colourful pages, which is much easier on the eye than pages and pages of text.
Fodors travel guides
Fodors, another well respected guide book, as with most here, specific countries are the norm, but they also do specials on National Parks and certain cities.
Click on the links to go to Amazon to check them all out, you can even look inside some of the books, to give you a small idea of what to expect, I'm sure you will find one to suit your tastes, and even find some bargains buying second hand ones!
Also take a look at the free online
World Travel Guide
And Ordnance Survey Maps