This is great beginner guide to identifying butterflies and some of the caterpillars to be found in Oregon:
Butterflies of the Pacific Northwest, by William Neill (Missoula, MT: Mountain Press Publishing Company, 2007), 208 pp.
Why do butterlies sometimes congregate in one spot? Can they see you? Are they eating that mud? Is it okay to hold a butterfly? Is it possible to raise them? Authoritative, easy to use, and downright beautiful, Butterflies of the Pacific Northwest answers these common questions and provides a comprehensive yet enjoyable introduction to these marvelous insects. The book is also a field guide to the 118 most commonly encountered butterflies in the Pacific Northwest-defined here as Washington, Oregon, and contiguous parts of California, Nevada, Idaho, and Canada. The clear, precise text and abundant color photographs allow for identification of both male and female adults as well as caterpillars, pupae, and egg. Author William Neill leads you to the best places to see checkerspots, fritillaries, coppers, blues, and many others, noting which plants attract them. Collectors of Pacific Northwest natural history will want to add this to their bookshelves.