Pat Draeger's request is a good one. There are quite a few books and
monographs on the freshwater/terrestrial mollusks of particular river
systems, states, or other geographic areas. Does anyone have a list of,
say, the 50 or 100 most useful manuals (as opposed to the 1000's of
specialized references in databases)?

Here's one:

Walker, Bryant, 1928, The terrestrial shell-bearing Mollusca of Alabama:
Geological Survey of Alabama, Alabama Museum of Natural History, Museum
Paper no. 8 (= University of Michigan, Museum of Zoology, Miscellaneous
Publication no. 18), 180 p., 277 fig.

Andrew K. Rindsberg
Geological Survey of Alabama