This was one of the first plant books I ever bought new. Looking back, I think it was a gateway book to quitting my job and going back to school for botany, and it is still one of my most frequently used botanical references. Matt Ritter is a botany professor at Cal Poly, but A Californian's Guide to the Trees Among Us is very accessible for non-plant people (color photographs! non-technical language! beautiful layouts!) while remaining highly useful to working botanists.
Pluses:
1. Excellent coverage of common ornamental trees planted in California. This book will cover about 90% of the trees you'll see in California neighborhoods or landscaping, and Ritter does a good job of including both NorCal and SoCal.
2. Great representative photos. You will instantly recognize many of these trees just from the well-chosen, full color photos included in this book. It's an attractive book to flip through, which I do sometimes when my expensive mapping software with every feature except autosave has just crashed. Recently, while driving, I identified an ornamental tree that is common in SoCal but I'd never before seen here just from its photo in this book. Any resource that enables drive-by-botanizing at 65mph in pre-dawn lighting has to be pretty amazing.
3. Dichotomous keys! I assume Ritter wrote these himself, and for any serious botanist, they are worth the price of the book. There are species-level keys for elms, maples, hackberries, and eucalyptuses, among many others, and they are remarkably clear and non-finicky. It's actually very difficult to find keys that include both native and ornamental species in California, yet I frequently need to be able to make the distinction between native and non-native trees within the same genus.
4. Organization. There are sections for gymnosperms and angiosperm dicots and monocots; within each, species are arranged by genus, which makes it easy to flip to the right page. (Plant books organized by common name are a headache - is it under dogwood or flowering dogwood? Ironbark or red flowering gum?) The index is also comprehensive and thorough.
In my botanical work along urban waterways, I often come across mystery ornamental escapees, and A Californian's Guide to the Trees Among Us has been invaluable for figuring them out. I'm raising my rating to a 5 because it is so useful and so thoughtfully put together. It's earned a permanent spot on my desk.