The following reference books have been invaluable to me for identification of animals encountered while scuba diving in the Pacific Northwest. All of these publications I use when looking up a new find, but Marine Life of the Pacific Northwest by Andy Lamb and Bernard P. Hanby is an overall excellent reference book that covers the widest range of life encountered.
Gregory C. Jensen, Daniel W. Gotshall, Rebecca E. Flores Miller. Beneath Pacific Tides: Subtidal Invertebrates of the West Coast. Bremerton: MolaMarine, 2018
Paul Humann with Photographers Howard Hall and Neil McDaniel. Coastal Fish Identification: California to Alaska. Jacksonville: New World Publications, Inc., 1996.
Andy Lamb, and Bernard Hanby. Marine Life of the Pacific Northwest: A Photographic Encyclopedia of Invertebrates, Seaweeds and Selected Fishes. Madeira Park: Harbour Publishing, 2005.
David W. Behrens. Pacific Coast Nudibranchs: A Guide to the Opisthobranchs Alaska to Baja California. Monterey: Sea Challengers, 1991.