The Complete Caves of Mendip and a view of the Hills (1977) by Willie Stanton & Nick Barrington
and
The Limestones and Caves of the Mendip Hills (1977). D. I. Smith and D. P. Drew
are still, in my opinion, the most up to date books that give, between them, the most information generally on the Mendip caves and karst.
Remember that they are both 35 years out of date now, but mostly the information within is reasonably accurate and still very relevant.
I would hope that there are plans to update the the Limestones and Caves of the Mendip Hills book and the (promised in 2013) new Mendip Underground is supposed to be the most comprehensive yet. Until then these two, between them, are still the best reference source on the caves, and should be in every Mendip caver's library.