OBNB doesn't sell books, but you may be able to find a copy at one of these websites:
Ffotograffiaeth : cyfrol ar hanfodion trin camera a chynhyrchu ffotograffau gyda chyfeiriadau gwyddonol i esbonio lle bo hynny'n briodol by Iwan Bryn Williams. ISBN 0901695211. Published by Llyfrau'r Faner in 1974. Publication and catalogue information, links to buy online and reader comments.
