Speleobooks is the most well-known vendor that sells caving books that I know of. They are based out of Schoharie, New York and sell their books at the Spring NRO & Fall NRO (usually both held in New York). They also sell their books at OTR (in West Virginia) and wherever the NSS Convention is being held (in 2020 it happens to also be at the same location as OTR in West Virginia).
There is also, of course, the NSS Bookstore based out of the NSS Headquarters in Alabama but they don't come to any caving events to sell any of their books that I am aware of.
The caving books that I am interested in are the ones that are essentially a guidebook to the caves of certain areas. In the Northeast, there are caving books that describe the caves of most states including Caves of Maryland (1971), Caves of Massachusetts (1969), Caves of New Jersey (1976), Underground Empire: Wonders and Tales of New York Caves (1966), Caves of Pennsylvania (1953), and Caves in Vermont (1959). West Virginia and Pennsylvania have books that specify the caves by county. So for West Virginia, there is a caving book describing the caves of Randolph County, West Virginia for example. And for Pennsylvania, there is a caving book describing the caves of Westmoreland County, Pennsylvania for example.
The NSS Convention Guidebooks are also a very handy resource to have. There was an NSS Convention in West Virginia in 2012 and in Pennsylvania for 2013.
NSS Bookstore:
https://members.caves.org/store/ListPro ... tid=669867Speleobooks:
https://speleobooks.secure-mall.com/Caves in West Virginia by County:
https://www.wvass.org/Caves in Pennsylvania by County:
https://caves.org/region/mar/bulletins.htm