"Thirteen" is a historical novel based in Caer Lludd (London) at the beginning of the Iron Age. It is based around the Celtic legend of the Thirteen Treasures of Britain. Queen Rhiannon of the Catuvellauni tribe and her estranged daughter separately have portentous dreams which tell them of the need to collect the treasures to save the tribe from war: in the first instance, the augury came too late to the nine-year-old Rhiannon the elder to be of use in the prevention of a battle, However, through the intervention of the Druid Dynedd, that first battle was won, but only just. The next battle,...