Dragonflies and Damselflies of Oregon: A Field Guide Kerst, C. & Gordon, S. (2011, OSU Press). This 304 page book sets a new standard for the identification of the 91 dragonflies and damselflies in Oregon. If you don't buy any other book be sure to get this one. The photos of male and female specimens are clear and large. The text gets the concepts across with a minimum of technical terms. Among the unique features of this book are identification charts for difficult families that incorporate both diagrams and text -- this proves a more effective approach than using either alone. A 16 page section describes 30 good places around the state to find dragonflies with notes about the species to expect at each location. It is slim enough to carry in the field. Given that the species list for Washington is almost identical to Oregon, this book will serve almost as well there.
Dragonflies and Damselflies of the Willamette Valley Gordon, S. & Karst, C. (2005, CraneDance Publications). The authors wrote this earlier 110 page guide as a fund raiser for the West Eugene Wetlands but is very hard to find now. It covers the 52 species known to the Willamette Valley of Oregon (Portland to Eugene). It includes identification tips and a short natural history for each species, including flight times. The photos are large and clear though often are posed using chilled specimens. This is an excellent reference for the Willamette Valley or Puget Trough.
Dragonflies and Damselflies of the West. Paulson, D. (2009, Princeton University Press).
This is the "bible" on Western dragonflies. Dennis Paulson has covered all the Western species with an extensive narrative and good (though small) pictures. He gives a basic description, identification issues, natural history, habitat, flight season, and distribution. This is the first guide to the region that includes a range map for each species. He also provides little tables of key characteristics of each specieis for difficult groups -- I like this much better than than the dichotomous key approach. At 535 pages, though, it is not something you will want to carry in the field. I recommend making a little index for fast reference to the pages for your local species of each family.
Dragonflies of Washington. Paulson, D. (1999, Seattle Audubon Society).
In this very slim volume but Dennis Paulson packed lots of useful information into a very consise format but it is no longer readily available. Although he restricts attention to Washington dragonflies, most of the species occur in Oregon as well. The photos are generally large enough to see the key features of a species. The narrative describes key features to distinguish similar species. He includes brief notes on range and habitat but no range maps.
Dragonflies Through Binoculars. Dunkle, S. (2000, Oxford University Press).
Comprehensive guide to 307 species found in North America in the style of the Butterflies through Binoculars book. Includes range maps, descriptions, and photos. Unfortunately, damselflies are not covered and the photos are generally too small to show much detail for my aging eyes.
Beginner's Guide to Dragonflies. Nikula, B., Sones, J., Stokes, D. & Stokes, L. (2000, Oxford University Press).
Good introductory guide to the more common species found in North America. The large photos are excellent and range maps are included next to the photo. However, only 44 of the species found in Oregon and Washington are included.
Dragonflies Berger, C. (2004, Stackpole Books). This little book includes a more complete description of the life history of dragonflies than found in identification guides. It includes a description of selected dragonflies but many are not found in this area and the guides above are most useful for identification.
A Dazzle of Dragonflies Michell, F.L. and Lasswell, J.L. (2005, Texas A&M University Press). A large format book on dragonflies. It covers a number of topics not covered in the Berger book.
Dragonflies: Behavior and Ecology of Odonata Corbet, P.H. (1999, Cornell University Press). This is a very comprehensive guide to the life of dragonflies. It describes key studies of dragonfly behavior in each phase of the life cycle. This is the ultimate reference on the subject but note that it is a scholarly work written for a technical audience. It is also out of print and thus rather expensive.