Good maps of Bangkok seem hard to come by. Before we left, we looked in several bookstores and decided on The Rough Guide to Bangkok. We'd never used a Rough Guide before but our usual (excellent) guidebook series, DK Eyewitness Guides, did not have a Bangkok-specific book, only a general Thailand guide. What a mistake...I've wanted to throw the RG right into the river about 10 times in the past few days. Meg promises me that once we get home, I can ritually set fire to it and cleanse ourselves of its crappiness.
On one of our last days here, we happened upon the Eyewitness Guide for Thailand and while it's thick and heavy, the Bangkok section would have been perfect for our needs. Argh! Oh well...one of the difficulties in traveling is that you never know what you're really going to need until you get to where you're going, and that goes double for maps.
We ended up relying quite a bit on the free SkyTrain/Metro map they give you at the station, as well as a slew of free maps available at our hotel and various other places around town. None of them was very good, but depending on what we were doing, one of them had the appropriate information on it. After all this, I wonder if a good map for Bangkok even exists[1]. The city is so big and sprawling that it's conceivable that no one has undertaken the effort to map it all.
[1] To its (possible) credit, the RG recommended a Bangkok map called Nancy Chandler's Map of Bangkok. We found it in a small bookshop on our last full day here, and while we couldn't properly evaluate it in its wrapper, it looked promising.