It will be your Bible. If you're really traveling independently, you'll need a guidebook that tells you how to survive, not just where the fun stuff is. Not all guidebooks are created equal. Some cater to independent travelers, and some cater to tour groups, and then some cater to the filthy rich. Also, it's not always easy to tell the difference. All guidebooks will have an overview of tourist sites, a few maps, some history, and some hotels. The difference is in the other stuff, which is actually the important stuff.
You want a guidebook that tells you how to get around with public transportation. You'll want to know where to find inexpensive but good food. Where do you go if you get hurt? A good guidebook will also tell you that. Most importantly, you want honest reviews of hotels and other places instead of reviews of companies that have paid to be included in the guidebook.
It is also important to get a recently published guidebook. Prices and quality change, and businesses go away and new ones take their place. We were once in China looking for a restaurant that was recommended, and when we got to the address, the entire block had been wiped clean and a new skyscraper was under construction in its place. Most guidebook publishers release new versions every few years, sometimes more often.
We do not endorse or have any business deals with any of the following publishers. All of these descriptions are from our personal experiences.
Lonely Planet has become the most popular guidebook publisher amongst "backpackers" and independent travelers with a budget-travel mindset. The books are usually excellent, and include the information that you want and need (how to get around on public transport, honest reviews of cheap hotels and restaurants, emergency information, etc.). Lonely Planet was started by travelers who blazed new trails on their own before any guidebooks existed for those places. In recent years, the company has branched out into more upscale markets, much to the backlash and complaint of the traditional budget "backpacker". They even have a TV show now. The "Thorntree" forums on their website are a very popular source for information. Despite the popularity, many travelers complain that once a hotel is included in a Lonely Planet guide, it gains a monopoly and then prices go up while quality goes down. Because of this and their recent upscale growth, many travelers now choose an alternative guidebook. On the whole, the books remain of good quality, and we recommend them.
Rough Guide and Let's Go are two publishers of guidebooks that cater to budget-conscious independent travelers. Both are very thorough. Like the Lonely Planet, they will tell you the information that you need to survive instead of just the history and glamour of the tourist sites. We highly recommend both.
Eyewitness Travel Guides are full of photos and colorful maps. If you're taking an all inclusive tour, this is the book to get. But if you need to know how to get around and where to eat and sleep, get a different book. National Geographic Traveler publishes guidebooks of a similar nature: beautiful with interesting information, but not always practical.