OBNB doesn't sell books, but you may be able to find a copy at one of these websites:
Cae'r Gors: Canolfan Dreftadaeth Kate Roberts = Cae'r Gors: Kate Roberts Heritage Centre. Published by Cae'r Gors: Canolfan Dreftadaeth Kate Roberts in 2009. Publication and catalogue information, links to buy online and reader comments.