Heart of the Dragon by Gena Showalter — book slip

Grace Carlyle is on a desperate search for her missing brother when she stumbles upon a hidden world filled with mythological creatures.

In this secret realm, she encounters Darius en Kragin, a shape-shifting warrior sworn to protect the gates of Atlantis. Darius is bound by a vow to kill any intruder, but Grace's presence challenges his ancient loyalties.

As they navigate the dangers of Atlantis, Darius must choose between his duty and the woman who has captured his heart.

First in the Atlantis series by Gena Showalter.