"The Meeting of the Tracks - Rock Art In Ancient Wales" - John Sharkey (2004 Gwasg Carreg Gwalch).
Handy book on Welsh rock art, as far as I can tell not an area well-covered otherwise (even Stan Beckensall generally seems to avoid Wales, except for "biggies" like Bryn Celli ddu).
Includes recent (up to 2004) discoveries by C-PAT.
pp148, b&w illustrations throughout. Barclodiad y Gawres, Bryn Celli Ddu, Ty Illtud, Llywel and Blaenawen have their own chapters, also a good site-list/gazetteer of 45 Welsh rock art sites.
Includes some post-Roman material.