Nancy Chandler's colorful maps of Bangkok pack loads of information into a very compact format. Shoppers will find the maps especially useful, as Nancy points out where the best buys can be found. Among the guidebooks, I'd rate Rough Guide's compact but thorough Bangkok guide as the best bet.
While a printed guide can be useful for helping you to plan a trip, they can often be bulky and awkward to take with you. Although some of the major guides are available in electronic format, they generally aren't organized in a way to be useful on the ground. My own electronic guidebooks have been specifically designed for use while you travel. They contain information from this on-line guide about each sight, what's nearby, and how to get there, all in formats for most ereaders or ebook apps.
All of the books below are available from Amazon.com, the worlds largest bookseller.