Guide to Marine Invertebrates: Alaska to Baja California, 2nd edition (revised)

This is an excellent guide to the invertebrates of the pacific coast. Make sure you get this updated 2nd edition, the older version (though still useful) is getting a bit dated. The pictures are fantastic, and identification is easy. Not all species are shown, but most of the critters I see underwater are illustrated in the book. There is one other book I like even better, but it costs more and isn't quite as small/portable: Marine Life of the Pacific Northwest: A Photographic Encyclopedia of Invertebrates, Seaweeds And Selected Fishes. That book contains similar high-quality pictures, but a lot more of them, and includes organisms besides invertebrates (nice algae/kelp id section).