[StephenTh]

Just re-read the original post - by "essential" do you mean "comprehensive"? If so, there are so many species that it's just about impossible to have one good book that covers everything. The nearest I know of is probably "Handbook of the Marine Fauna of North-West Europe", by Peter J Hayward and John S Ryland, ISBN-10: 0198540558. - link to one supplier here. This book is an identification key, and doesn't really cover things like behaviour. Even this book isn't completely comprehensive - and it's a bit of a dry read! You might want to have a look at a copy first - universities should have it in their library if they do marine biology. For the vast majority of the things we're likely to see, the books already mentioned are fine. Best of luck.