If you're just hitting Beijing for three-four days, stick with Frommer's, Lonely Planet et al. But if you're planning a longer trip, and most especially if you're a truly curious traveler interested in not only the what but also the WHY of a historical but living and fast-transforming city, the Insider's Guide to Beijing will be your trusted companion. It's a great guidebook while you're in the city but also makes fascinating pre- and post-trip reading, with great nuggets of info on how the taxi cab license plate system works, interviews with local people like chefs at famous restaurants, and humourous but also useful info on topics like "breathing in Beijing" and how to get from place to place using the fastest mode of transportation.
The fact that the Insider's Guide is really written by insiders - long-time expat residents who clearly know and love the city but also understand all its quirks, means you also get the benefit of informed reviews of everything from bars to shopping areas that go far beyond a thumbs-up/thumbs-down rating to really give you a sense of whether YOU as an individual would enjoy it. Since the writers are long-timers, you get a sense throughout the book of the fast changes Beijing is going through as well.
Other than the fact that it could use a few more maps (easily available at your hotel or a shop, however) it's a terrific, jam-packed, informative and easy to use guide to one of the world's great cities.