Here some books I use frequntly,

Recent European marine molluscs

Synopsis of the British Fauna (New Series)
No 2: Molluscs: Prosobranch and Pyramidellid Gastropods, Alastari Graham,
F.R.S., 2nd Edition, 1988
No 8: Molluscs: Benthic Opisthobranchs, T.E. Thompson, 2nd edition, 1988
No 37:: Molluscs: Caudofoveata Solenogatres, Polyplacophora and Scaphopoda,
A.M. Jones and J.M. Baxter, 1987

N. Tebble, British Bivalve seashells: good drawings but no synonyms

West European land-molluscs

Elseviers Slakkengids, a Dutch translation of A field guide to the Land
snails of Britain and North-west Europe, M.P. Kerney & A.D. Cameron, 1980

Dutch Fresh-water molluscs:
Nederlandse fauna 2: De Nederlandse zoetwatermollusken.
Deals with the recent and quaternary fossil molluscs from fresh and brackish
water, with magnificent illustrations, text in Dutch.

For the determination of Middle Miocene molluscs from The Netherlands:
A.W. Janssen, 1984: Mollusken uit het mioceen van Winterswijk-Miste. Deals
with 500 fossil species, every species is illustrated, but the text is in
Dutch.

Martin C Cadee,
The Netherlands