Posted 16 August 2009 - 05:38 AM
best out there (which isn't saying much) are:
Burch, J. 1982. North American freshwater snails: Identification keys, generic synonymy, supplemental notes, glossary, references, index. Walkerana 4:1-365.
Burch, J. 1988. North American freshwater snails: Introduction, systematics, nomenclature, identification, morphology, habitats, distribution. Walkerana 2(6): 1-80.
Keep in mind that much of our understanding of diversity and systematics has changed since these came out...
There's also a couple folks at the Field Museum that are actively working on fw gastropods of IL; ask Uland for Stephanie's contact info and see if she can provide you with some more direction.
hope this helps.