Hi Mark,
I have some dozen such books in Ohio. Most of them are regional such as
"Unionidae of Ohio" published by Ohio Dept. of Natural Resources (or something
like that)
 
The Book Guy (I can't remember his name) in North Myrtle Beach has a wide
selection of used books on Unionidae.
 
Sorry I am away from home and don't have good references and my memory is
getting selective.
 
Oldsheller    Bud Rogers