I have (quite happily!) spent two great vacations in Napa and Sonoma Counties. This is the best guidebook I have found for that region.
There are a few things that make this guide very helpful. First, the author makes sure this guide is very user-friendly for first-time visitors. In other words, you get a true introduction to the Northern California area and what's special about it. You also learn which highlights are worth hitting, and what's worth ignoring.
Second, the visual aspect of the book is great: super-colorful, great layout, things rated from one to three stars, lots of little neighborhood maps, color photographs, etc.
Third, it's clear the author knows what she's talking about at all times.
Fourth, the book comes with a little map that--although it looks a little geeky--is actually quite helpful for finding wineries as you drive along.
Fifth, there's a nice little guide to San Francisco that is the last chapter. It's very well done.
Caveats: if you are a Wine Country veteran, you are too advanced for this book and should skip it. Also, you'll need a road map (such as the paper folding ones that you can buy in a gas station) as a supplement to the maps in this book, because the book's maps don't get the job entirely done.