Mark of the Sylph (Demons of Infernum #2)

Half-demon Taeg is desperate to free his pregnant sister-in-law, who is spellbound to an indestructible book that will create an army of the undead. He is convinced that an ancient sword can destroy the book and the spell forever, but this relic has been hidden for a millennia by a powerful glamour. The clock begins to run out on his search when an old rival goes after the book and Taeg’s family in his quest for revenge.

Librarian Maya Flores has a secret: she’s immune to magic and is well aware of the supernatural beings prowling the streets of New York City. Haunted by a brutal demon attack that took her family, she is on a one-woman mission to destroy the evil demons who did it—and any others who get in her way.
Maya is Taeg’s best hope for uncovering the sword.
Unfortunately, she wants nothing more than to kill him. As he fights for her trust and to save his family, she is forced to face the truth that not all demons are evil. And the worst part? She might be falling in love with one, too.