There are 3 books in the Welsh Princes Trilogy series. The newest book is The Reckoning which came out in 1991.

1
Here Be Dragons

Book cover of Here Be Dragons
An absorbing historical novel of power and betrayal, loyalty and political intrigue in thirteenth-century England, Wales and France, centring on King John of England, younger brother to the brilliant Richard Lionheart, Joanna, his illegitimate but recognised daughter and Llewellyn Ab Iowerth, Prince of Gwynedd, a bitter opponent of English ways,…
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2
Falls the Shadow

Falls the Shadow
In thirteenth-century England, King Henry III's sister Nell breaks tradition and marries Simon de Montfort, an outspoken nobleman who risks death in battle to preserve honor

3
The Reckoning

The Reckoning
This book completes the splendid sequence of novels on the struggle between the independent Welsh Princes and the growing English strength which began with Here be Dragons, continued with Falls the Shadow and is now completed with The Reckoning. The major figures in The Reckoning are the splendid dominant King…