Hi there, have done quite a bit of research on mining in Somerset and its links to Cornwall as I live there now, several minerals were mined there at one time or another, not just coal..copper, silver, iron, lead, calamine (zinc), celestite and maybe more
for info on the coal mines your first port of call should be Down & Warringtons excellent 'History of the Somerset Coalfield' (reprinted 2005), and for photos of Kilmersdon and neighbouring collierys I can recommend John Cornwells Collieries of Somerset and Bristol, some of the best photos out there I think!