The Thorn and the Spire by Katherine McIntyre

One lonely demon. One curious librarian. One tale twisted by time…

Beau’s father is missing.

The trail leads to the Spires, a casino with a dangerous reputation run by Cillian Ashmore, a demon whose ruthlessness is renowned. His father landed himself a ten-year imprisonment, but Beau offers to take his place. Yet Cillian is as cold and callous as he’s rumored to be, prepared to make Beau’s time as his personal assistant hellish.
The mysteries in the upper tiers of the Spires multiply by the day, and the further Beau explores, the more questions he has. Why sometimes the rooms are empty and other times monsters lurk within. Why Cillian seems cruel yet has the friendship and loyalty of those around him.
And the more Beau discovers, the more he learns that Cillian’s cruelty is a mask he dons, that the demon has depths he’d never dreamed of. Too easily, that loathing morphs to lust and to something intense and unforgettable.

Yet Cillian’s made himself dangerous enemies, true threats lying in wait beyond the Spires, and unbeknownst to Beau, he’s running out of time…