The Sutherland Devil (The Sinclair Jewels 3) by Caroline Lee

She'll become a squire to search for the Sinclair jewels…even if her new master is the very Devil himself!

Everyone in the Highlands knows three things about the Sutherland laird:
1) He's got more bastards than he can count.
2) He's already buried three wives.
3) He's the very Devil himself.
Which is why, when Saffy—one of the Sinclair Jewels—finds evidence that her family's missing brooch is in the Sutherland Devil's possession, she's certain she's taking her life into her own hands to reclaim it. She's the scholar in the family, not the warrior…but how hard could it be to don a pair of braies, lop off her hair, and approach the Sutherlands as a lad?
She hadn't planned on being accused of spying and thrown into the dungeons to starve!