Sacred Circles: Prehistoric Stone Circles of Wales - Julian Heath (2010, Llygad Gwalch)
Useful general introduction to Welsh stone circles, featuring 34 of the stone circles in the country, from the well-known ones like Cerrig Duon, Penmaenmawr, etc to some very obscure ones - including Llorfa, which I'd never even heard of. It aims to "encourage you not just to read about the circles, but to go out and visit them". Sounds like a winning statement of intent.
136pp and several pages of colour photos, a neat gazeteer including the author's own experiences, perhaps rather too honest in places:
Crugiau Bach - This circle is located above the village of Llanwrthwl, near Llandrindod Wells, but I am not sure how one reaches it.
Also includes a decent bibliography for further exploration and namechecks a few websites - including this one!
All in all well worth picking up. Bought new for £7.50 in Cardiff today.