Summary from Goodreads:

Gia Kearns would rather fight with boys than kiss them. That is, until Arik, a leather clad hottie in the Boston Athenaeum, suddenly disappears. While examining the book of world libraries he abandoned, Gia unwittingly speaks the key that sucks her and her friends into a photograph and transports them into a Paris library, where Arik and his Sentinels—magical knights charged with protecting humans from the creatures traveling across the gateway books—rescue them from a demonic hound.

Jumping into some of the world’s most beautiful libraries would be a dream come true for Gia, if she weren’t busy resisting her heart or dodging an exiled wizard seeking revenge on both the Mystik and human worlds. Add a French flirt obsessed with Arik and a fling with a young wizard, and Gia must choose between her heart and her head, between Arik’s world and her own, before both are destroyed.