Angles; It is rare you will catch the specific identification feature you need in one single shot, the list below gives the key points for different organisms.
The best books are
The Sea Search series (anemonies, seaweeds, bryozoans and now sea squirts). Available on Amazon.
The Student’s Guide to the Seashore by Fish and Fish and the Collins Guide to the Seashore.
The best websites are
Facebook groups such as Rockpooling and shrimping,
Coastwise North Devon and of course Your Shore network.