The Iron Khan — Liz Williams

A supernatural mystery set in Singapore featuring a “detective whose beat reaches to the fringes of Heaven and Hell,” by the author of Snake Agent (Booklist).
 

No mortal has ever heard of the Book, and few in Heaven even believe it is real. Instead, they regard the stories of a bound volume older than time itself as something of a creation myth. But Mhara, the Emperor of Heaven, knows the Book is very real, very powerful, and very much missing. It has a mind of its own, and it appears to have wandered off -- taking the secrets of the universe with it.

To find it, Mhara calls Detective Inspector Chen, a supernatural sleuth with previous experience in saving the universe. Chen has a lot on his plate at the moment: His wife is pregnant, his demonic partner is tracking the movement of an immortal horde, and he hasn't had a vacation in years. But for the sake of the Emperor, he'll do his best to return order to the cosmos. If he doesn't, who will?