"Prehistoric Sites of Montgomeryshire" - Beth McCormack (2006 Logaston Press).
Volume XI in Logaston's excellent "Monuments in the Landscape" series.
Includes detailed background chapters on Stone, Bronze and Iron Age in Montgomeryshire and surrounding areas. Details of over 30 principle sites, with access details, some plans and photos. 163pp.
A number of the earlier volumes in the series are out of print, but they are well worth seeking out.