Once Upon a Midnight Kiss by Elisa Braden

Every dream job has a catch
Housekeeper Adeline Black’s new position seems too good to be true. Dusting a collection of peculiar relics? Too easy. Having a splendid house all to herself? Too lovely. Addy’s not about to question why she was told to never open the library door. Or why there’s a mysterious portrait of a medieval knight locked inside. But libraries need dusting, too. And what’s the harm in chatting with a knight over tea? It’s not as if he’s real. It’s not as if she’s falling in love.

Every portrait has a story
Sir Hugh Marshal is very real. Unfortunately, so is his curse. Trapped in an eternal prison, he’ll take whatever distraction he can get, even if she’s a bookish housekeeper with a stubborn streak and a vexing sense of whimsy. Even if he might be falling in love.