Handbook of the Marine Fauna of North-West Europe

This is an invaluable book that I regularly use.

The Handbook of the Marine Fauna of North-West Europe does what it says on the cover, and the Amazon reviews reflect its usefulness:

When Peter Hayward and John S. Ryland first published Marine Fauna of the British Isles and North-West Europe, it became an instant classic in the marine reference literature. Now with Handbook of the Marine Fauna of North-West Europe, the same editors offer a concise, practical guide to over 1,500 species from the major marine phyla-from sponges to fish-in a format that is ideal for field use. With its simple dichotomous keys, individual descriptions, profusion of illustrations, and extensive reference section, the book allows for rapid and easy identification of all but the rarest marine animals found on the sea shores and shallow sublittoral zones of the region. Students, researchers, and amateurs interested in zoology, marine biology, and ecology will all want to own a copy of this unique field guide.

“The essential guide for any aspiring marine biologist”, 16 Dec 2011
By Magnus Johnson (UK):

This is a core text that we use at the Centre for Environmental and Marine Sciences for our undergraduate courses in Marine Biology and Ecology. It is “the book” to use as a starting point and it is generally enough until you get to postgraduate level or need to specialise in a particular group of animals (it doesn’t cover algae obviously). For each group there is an introductory section that gives you a guided tour of body parts. This is usually followed by a key to families which is generally easy to follow once you have the taxa specific nomenclature in your head. Handy references to diagrams direct you examples of each family. Once you have identified your specimen down to family there is a key to species. For most taxa each species has a succinct description with details of their distribution and colouration that allows you to confirm your identification.