This is an original synthesis of fantasy and historical novel. In an alternate Middle Ages, there's a species of merpeople who are sufficiently closely related to the landsmen to be able to interbreed. The infant Whistle, a sea-dwelling outcome of such a union, is turfed out of his tribe by his mother, washed up on the shore and taken in by a landsman who renames him Henry.
As he grows, he learns that he's being groomed as a bastard pretender to the throne of England – only half-breeds like himself can be royal, and the present king is old, with no suitable heirs. Henry's story converges with that of the princess, Anne, who is desperately trying to secure a stable succession. It's a brilliantly realised picture of a medieval England that never was, and a Machiavellian meditation on what must be done to seize, wield and maintain power.
Sadly, the second half of this novel is nowhere near as good as the first. The pace slows, the tension slackens and the storytelling becomes increasingly repetitive, with an irksome insistence on points the reader already knows.