Jane Grigson's "Fish Book" is extremely comprehensive and a good read in its own right. I'd say that it assumes a fair bit of competence though and the paperback edition isn't easy to lay flat and doesn't have any photography.
Sophie Grigson's (daughter of Jane) "Fish" is the fish book I've used most, but may not be in print at the moment.
Other than that, Rick Stein's "Fish and Shellfish" was published last year and updates his earlier "Seafood". Haven't used it myself, but have used a couple of his other books and his recipes are usually pretty bombproof.
