This is book number 1 in the Library Jumpers series.
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.
The Library Jumpers series is best enjoyed in order.
Book #1 Thief of Lies
Book #2 Guardian of Secrets
Book #3 Assassin of Truths