Pegasus: The Flame of Olympus by Kate O’Hearn

When Pegasus crashes onto a Manhattan rooftop during a raging storm, Emily’s life changes forever. Caught between the immortal world of Olympus and the dark forces threatening to destroy it, Emily joins forces with a thief named Paelen, the goddess Diana, and a mortal boy named Joel to protect the Flame of Olympus. But as ancient enemies rise and powerful secrets unfold, Emily discovers she may hold the key to saving both gods and humans from total annihilation.