The Pretender by Egan O'Neill. 1st Edition, Hardback+Dust-Jacket, W.H.Allen 1957.

The author's actual name is Elizabeth Linington an American who wrote under several monikers, including 'Egan O'Neill' 'Anne Blaisdell', 'Lesley Egan',  and 'Dell Shannon.' 

The Pretender is the story of an 18th Century Irish lawyer Dennis McDermot who while posing as a friend of England played a double game in various treasonable anti-English activities. He also battles for the hand of an English heiress against his most astute opponent.

Condition.  Hardback with gold lettering in excellent condition. Some foxing but well preserved. Dust-jacket in protective sheaf, slightly worn.