I've used a whole lot of travel guides over the years. I've used The Rough Guide, Frommers, Lonely Planet... you name it. I know that a lot of things, in fact nearly everything, are available online these days. But the thing I've found is that I don't always have online access when I am on the road and wanting a little information at my fingertips. So, carrying a good travel guide is still necessary, for me.
What I have found, in using so many different guides, is that Lonely Planet's guides are the best hands down. Their information is the most thorough, giving specific information on each place to see, place to eat, or place to stay, including prices and sometimes small details that other guides seldom include.
If you're looking for a good travel guide, Lonely Planet is the one.