With a Little Help from my Lord by Cheryl Bolen

He must guard her—but can he guard his heart?

An innocent trip to the book store propels Lady Harriett Beresford into danger when a stranger—grabbing her book and warning of peril from the Fat Man—collapses on her doorstep. Dead. She rushes to find aid, and when she returns, the dead man is gone, and no one believes her account of the fallen book thief.
Alex Muir, Baron Rockingham, has been charged by Devere, his closest friend, to watch out for his sister while he’s away on his wedding trip. When Alex stops by to check on Harriett and she explains her predicament, he’s skeptical. His friend’s exuberant little sister has always had a flair for the dramatic. But when she later turns to him following a harrowing attempt on her life, his doubts are put to rest. She has been telling the truth all along, and he now knows her life is in peril. His protective instincts kick in. He cannot allow her to return to her home. She must stay with him so he can guard over her at all times.
Together, they seek the identity of the Fat Man, and in doing so, they learn of a plot against the regent. As important as it is to Alex to protect the regent, it’s even more important that he keep Harriett safe. Spending so many hours a day with her—and so close to his bedchamber—Alex comes to realize Devere’s youngest sister is no longer a girl. She’s a young woman, a desirable young woman.
But he cannot give in to his searing desire. All that matters is protecting the life of this woman who has come to mean so much to him.