OBNB doesn't sell books, but you may be able to find a copy at one of these websites:
Castell Henllys : caer o'r Oes Haearn : llwybr hunan-dywys = Castell Henllys : Iron Age fort : self-guided trail. Published by Pembrokeshire Coast National Park in 1992. Publication and catalogue information, links to buy online and reader comments.