A Famine of Horses — P.F. Chisholm

The author, who under her real name Patricia Finney has published two well received historicals, The Firedrake's Eye and The Unicorn's Blood, both set in the rein of Elizabeth I, parlays her mastery of the period with a robust sense of humor and vividly conceived characters to produce the American Western in Elizabethan Border Country. There is little difference between the Scottish reivers thieving horses and brawling with both each other and the hated English and the gunslingers who made Tombstone and Boot Hill notorious.
In the year 1592, Sir Robert Carey, a handsome courtier, comes north to Carlisle to take up his new post as Deputy Warden of the West March. He has wangled his appointment to be nearer his true love, a married woman, and farther from the still eye of his Queen and the disapproving eye of his father. And of course, there's the money....
Sir Robert is quick to realize he won't see a profit from the perks if he fails to keep the peace. Alas, he is quickly challenged by the murder of a local lad, the possible betrayal of a disappointed rival, the ire of the as-yet-unbetrayed husband, and the question of the horses--hundreds of horses being stolen from all over the lands lying between England and Scotland.
Rich in atmosphere and packed with real and fictional characters, few novels are as well imagined or as much fun as this romp through rustling, treason, and high ambition.
The other works in this series are A Plague of Angels (Introduction by Diana Gabaldon), A Season of Knives (Introduction by Dana Stabenow), and A Surfeit of Guns (Introduction by Barbara Peters). The author is at work on a fifth Sir Robert Carey.