A Traveller's Guide To Mars by William Hartmann

This book has the form factor and layout of a typical travel guide, which is a neat gimmick. Inside is a really interesting, detailed explanation of what we know about Mars, up to 2003 or so (when it was written). It also has a fantastic selection of photos, many from the Mars Global Surveyor.

Hartmann takes a tour by region, in each one telling you a little more about what we know (or speculate). The tour covers areas of interest - both the obvious places to laypeople (e.g. Olympus Mons) and those that are geologically important. He starts with the oldest parts of the planet and works his way to the youngest, overall.