Native Trees of Western Washington: A Photographic Guide, by Kevin Zobrist. 2014. An excellent photo identification guide. May be purchased from WSU Press, Amazon, or other book sellers. $18.95.
Trees of Idaho is a 24 page booklet that describes the native species of Idaho. Most of these species are also common in eastern Washington.
Silvics of North America. Volume 1, Conifers. Volume 2, Hardwoods. USDA Forest Service, Agriculture Handbook 654. 1990. These books describe the silvical characteristics of about 200 conifers and hardwood trees in the conterminous United States, Alaska, Hawaii, and Puerto Rico. Many topics are covered for each species, including habitat, climate, soils and topography, associated forest cover, life history, seedling development, growth and yield, rooting habitat, reaction of competition, damaging agents, special uses, and genetics
Common trees of the Pacific Northwest. This online resource from Oregon State University provides lists by common and scientific name, descriptions and photos of the species, and a dichotomous key for identifying plants that you don't know.
Arbor Day Foundation hosts a simple-to-use online identification site called What Tree is That?
The USDA Natural Resources Conservation Service PLANTS Database is a more comprehensive, more complex, online tool for plant identification. It provides standardized information about the vascular plants, mosses, liverworts, hornworts, and lichens of the U.S. and its territories. If you are familiar with plant keys, you may find this one easier to use than the usual dichotomous key, which has only 2 choices for each step. This online version allows the user to select a larger number of appropriate characteristics at one time. There also are lists by state and maps that show county distribution of different species.
The University of Washington and Burke Museum of Natural History and Culture host the WTU Herbarium site of plants in Washington. It includes the WTU Image Collection and a Random Access Identification Key to help you find the desired species. For each species, a description, distribution map, and images are included.