Mortal Coils is the first book in a planned five-book series. I'm to understand Nylund did in fact write books before he adapted the Halo series — but this is the author's first original work since he did Halo: The Fall of Reach, Halo: First Strike and Halo: Ghosts of Onyx.
The press blurb on the plot reads like half a dozen different manga I read in high school. Here's a tiny taste of it:
On the eve of their fifteenth birthday, however, everything changes. It begins with hauntingly familiar violin music played by a homeless man who reeks of sardines and sulfur; a victorious confrontation with their bully of a boss; and a visit from two mysterious strangers, one known only as "a Driver" and the other who claims to be their long-lost uncle.
It turns out that Fiona and Eliot are much more than ordinary teenagers. They are the result of a single mistake: Years ago, an immortal goddess…and the infernal Lucifer, the Prince of Darkness, fell in love. To protect them from their dangerous heritage, Audrey Post valiantly kept the twins hidden and camouflaged from the entities that have sought them over the years, transforming the divine into the dull.
Not to say anything bad about Nylund's writing (I actually liked Fall of Reach), but I'm betting the "New York Times Bestselling Author" tag he gets to put on Mortal Coils comes more from Master Chief's massive popularity than it does from his writing.