Burn After Reading by Catherine Ryan Howard

The night Jack Smyth ran into flames in a desperate attempt to save his wife from their burning home, he was, tragically, too late - but hailed a hero. Until it emerged that Kate was dead long before the fire began.
Suspicion has stalked him ever since. After all, there's no smoke without fire.
A year on, he's signed a book deal. He wants to tell his side of the story, to prove his own innocence in print. He just needs someone to help him write it.
Emily has never ghostwritten anything before, but she knows what it's like to live with a guilty secret. And she's about to learn that there are some stories that should never be told...