Well the Architecture of Death one by Geo.Nash can be found here, under general books on Wales.........

http://www.logastonpress.co.uk/

as can Neolithic Sites of Cards, Carms, & Pembs by G.Nash and G.Children, there was a sort of run of books under the 'Monuments in the Landscape' series.... Herefordshire, Monmouth etc.....
Prehistoric Preseli by N.P.Figgis is an excellent area book.
Probably the best advice is to look for Welsh books when you're in Wales, browsing bookshops is part of the holiday.