The better book about shells

[dave r]

That all rather depends what you want it for.
As a straight identification guide the Encyclopedia is very good with the latest nomenclature, the downside is it does not give distribution of species or size range so you don't know if the species you have is correctly identified as it looks like the picture but comes from elsewhere. It only gives the locality and size of the specimens figured.
The Compendium has been the shell collectors 'Bible'for many years, it is a bit dated now but it does at least give the distribution and sizes of the species illustrated plus a few other odd bits of information. It has chapters on collecting and shell biology and morphology which the encyclopedia does not. It has been out of print for a while now but overall I would go for the Compendium of seashells it as it has much more information and illustrates other shells such as bivalves and Chitons whereas the Encyclopedia of Marine Gastropods is purely a picture book and, as the title states, is purely Gastropods.
Hope this has given you some help,
Dave