This is an interesting--even fascinating--guide, with a great introduction that you won't want to miss. It covers a lot of territory, but its illustrations and keys are very easy to use. Petrides has developed a fool-proof system of identification that is intuitive and enjoyable.
As a lay-naturalist wandering for a week in the Bitterroot Mountains of Montana, I found this book an excellent companion. It's a joy to know the names of the trees; it deepens one's relationship with them and helps one appreciate their being. When you can call them by name, you get closer to them. This guide also includes extra information about each tree, as if the most fascinating aspects were condensed into each entry.
Recommended to Wanderers of the West, or anyone west of Minnesota with a tree in her back yard.
I lost my copy years ago - and then Kerri found me a new one. I'm such a sucker for conifers. (See also: this is what I do when I'm home sick. I read about places I'd rather be.)
[4 stars for all the beautiful nerdery and wooded dreams.]
This is my most frequently used field guide for identifying Western trees. The tabular information and color drawings included in the plates are very helpful for distinguishing between similar species (especially for conifers, willows, oaks, etc.).