I'd recommend The Architecture of Death by George Nash for the whole of Wales. If just Anglesey, I also have a little CADW guide that covers ancient monuments (altho some are modern (roman and medieval).

Another all Wales one I like is Christopher Houlder, Wales: An Archaeological Guide.

The Nash one is my fave even though it only covers chambered tombs - there are still a fair few of them in Wales, even if you ignore the rest!

Hope that helps

Sam