Flight of the Swallows — Stephen Cosgrove

From award-winning author Stephen Cosgrove, creator of the Serendipity series, The Diary of an Angel trilogy, his first adult novel.

When seventeen year-old Libby Brighton stumbles upon an archangel trapped on the Internet, she soon finds herself in a mysterious world that has her switching bodies with the angel and searching for trapped souls deep in the catacombs beneath the Old Mission at San Juan Capistrano.
With the archangel guiding her, the story climaxes with Libby battling the demonic Darkside Veil and its minions, the Changelings, to save the souls and rescue mankind.