His Master's Daughter (Royal Guardians #1)
Christina Rich

In exchange for his freedom, Ariel’s master offers him Sh’mira as a bride and the rights of a firstborn son, but the former Commander of the Temple Guard cannot accept the honor. He’s bound by a vow to protect Judah’s future king from the child’s murderous grandmother, but when the queen’s guard attacks the tiny village he’s been hiding in he finds he cannot leave his master's daughter.