A good general introduction to the biology of molluscs is provided by J.E. Morton's "Molluscs" published by Hutchinson & Co 1958.
Some of the books listed may be out-of-print, but second-hand ones may occasionally be found and those listed are considered useful by many. In adidition there are a range of resources available on-line.
Land snails and slugs
CAMERON, R.A.D. 2003. Land Snails in the British Isles. Preston Montford. FSC Publications
CAMERON, R.A.D., EVERSHAM, B.C. and JACKSON, N.C.S. 1983. A field key to the slugs of the British Isles. Field Studies, Volume 5, pp. 807 - 824.
ELLIS, A.E. 1926; reprinted with appendix 1969. British Snails. Oxford. Clarendon Press
KERNEY, M.P. and CAMERON, R.A.D. 1979. A Field Guide to the Land Snails of Britain and North-west Europe. London. Collins
QUICK, H.E. 1960, British Slugs. Bulletin of Br. Mus. (Nat.Hist.) Vol. 6. No. 3. pp.106 - 226
Fresh- & brackish-water bivalves and gastropods (includes Quaternary fossils)
GITTENBERGER, E., JANSSEN, A.W., KUIJPER, W.J., KUIPER, J.G.J., MEIJER, T., van der VELDE, G. & de VRIES, J.N. 2004. De Nederlandse zoetwatermollusken. Recente en fossiele weekdieren uit zoet en brak water. -- Nederlandse Fauna 2. Nationaal Natuurhistorisch Museum Naturalis, KNNV Uitgeverij & EIS-Nederland, Leiden, 288 pp.The 2004 print is the second edition, which has been slightly updated as compared with the first of 1998. The book is applicable to NW European (non-phreatic) fresh water molluscs (bivalves and gastropods), and includes the Quaternary fossils. In Dutch.
GLÖER, P. and MEIER-BROOK, C. 1994 (in German) Süsswassermollusken . Hamburg. Deutscher Jugendbund für Naturbeobachtung.
GLÖER, P. 2002 (in German) Die Süsswassergastropoden Nord- und Mitteleuropas. – Die Tierwelt Deutschlands 73: 327 pp., ConchBooks, Hackenheim.
MACAN, T.T. Fourth edition 1977. A key to the British Fresh- and Brackish-water Gastropods, Freshwater Biological Association. Scientific Publication No. 13
ELLIS, A.E. 1978. British Freshwater Bivalve Mollusca. London. Published for the Linnean Society by Academic Press
KILLEEN, I., ALDRIDGE, D. and OLIVER, P.G., 2004. Freshwater Bivalves of Britain and Ireland. Preston Montford. FSC Publications.
GRAHAM, A. 1988. Molluscs: Prosobranch and Pyramidellid Gastropods. London. Published for the Linnean Society and the Estuarine and Brackish-water Sciences Association by E.J. Brill / Dr. W. Backhuys, Leiden
TEBBLE, N. 1966. (Second edition 1976) British Bivalve Seashells. Published for the Royal Scottish Museum by Her Majesty's Stationery Office, Edinburgh
THOMPSON, T.E. 1988 Molluscs: Benthic Opisthobranchs. London. Published for the Linnean Society and the Estuarine and Brackish-water Sciences Association by E.J. Brill / Dr. W. Backhuys, Leiden
Other marine molluscs (Chitons, Scaphopods, etc.)
JONES, A.M. and BAXTER, J.M. 1988 Molluscs: Caudofoveata, Solenogastres, Polyplacophora and Scaphopoda. London. Published for the Linnean Society and the Estuarine and Brackish-water Sciences Association by E.J. Brill / Dr. W. Backhuys, Leiden