The Sutherland Devil 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!
Merrick Sutherland is harried from all sides: his alliances are falling apart, his children are adorable monsters, and his illegitimate brother has declared war in an attempt to claim his title. The last thing he needs is a distraction in the form of a lippy little cross-dresser, but that’s what he gets when he drags his new prisoner up from the dungeons. Is she a spy? The best way to learn her secrets is to keep her close, so Merrick makes “Saf” his squire, without revealing what he’s already learned.
Thrust into a sensual game of wits with her new master, Saffy is surprised to learn that what everyone knows about him isn’t quite true. As his squire, she’s in a prime position to search for the missing Sinclair jewels, and she’s willing to go through the very Devil himself to find them.
But will she lose her heart in the process?