In the preamble to the 2014 edition of his guide to Germany, Rick Steves explains that "German culture divides at a sort of North-South Mason-Dixon Line." Southern Germany is largely Catholic and relaxed while Bavaria is the source of the beer-and-pretzel stereotypes "probably because that was 'our' sector after the war." This is a simple insight simply rendered, but it is also an illustration of what separates Steves' books from Lonely Planet or Eyewitness tomes. He's a guy with an opinion trying to explain what's going on.
Steves is unafraid to step on toes or rib locals in the service of informing travelers. That's what makes him good at his job. Instead of describing Shakespeare's home town of Stratford as charming (as nearly all guidebooks do), Steves advises travelers to give it a miss. His goal is not to write a comprehensive book, but to serve as an able guide. It's an important distinction.
Most of travelers have – if they're lucky– a week or two of vacation. That's why they pay the $16 for a book that tells them they don't want to miss Sognefjord in Norway or Burg Eltz castle in Germany. But the real value is in finding a guidebook that tells you what you don't need to see. Modern travel is about triage. Now that everyone can go everywhere, smart travelers distinguish themselves by focussing their efforts.
It's all the more important to find well-written, opinionated guidebooks in the age of the internet. Want to know how to get somewhere in Costa Rica? Google it. Want to know if that somewhere is worth getting to? Read Christopher Baker's Moon Handbooks: Costa Rica. The guy has been writing about the country for the better part of 20 years while also serving as a National Geographic guide. He knows more than any anonymous contributor to WikiTravel.
Here are the three things to look for in a guidebook:
Publication Date: If you're heading to Chicago, you might be fine with something a bit outdated, but places in the developing world change rapidly and you don't want an old book in a place where you don't speak the language.
Author: If this is the author's first book, don't buy it. Look for an experienced author who lives in country. Guidebook authors are generally paid with a lump sum from which they extract expenses so someone local will almost always do a better job.
Negativity: If you read three pages and the author doesn't say anything critical about the destinations he or she is covering, the book isn't going to do you much good. Guides provide direction.
Generally, you'll find that the books that meet these criteria are either by Rick Steves or published by Bradt or Moon. These old-school companies may not be around for much longer (guidebook companies are dropping like flies), but they still make a great product. There are an increasing number of independently published titles that are best avoided and sometimes it pays to go with one particular brand. If you're going to a dangerous or obscure country, look for the Brandt book. The company works with serious writers.