My mother-in-law is self-publishing a book she has written, which is a fictionalised account of a real love affair which her aunt had with a famous Russian writer, Boris Pilnyak. She has taken letters which she discovered after the death of her aunt and woven a story around them.
She has been very pleasantly surprised at the number of orders she has received for the book, with scholars from Oxford and Harvard, among others, getting pre-orders in.
My question is; how much utility does a book like this have to an academic historian? I always imagined that good modern history books would be more a case of "just the facts, ma'am".
Based on my experience, what you're talking about would more likely be used in English or literature classes rather than in the history department. Not one of my history classes included readings factoring in a fictional story-typically it's more 'purely factual,' but enlightened with anecdotes and the like to flesh the book out and make it more interesting.
The biggest shift I can think from this trend was my ancient Greece class, where we were assigned Homer's Odyssey and had to read the book with the understanding that the eclipse mentioned in the final book places the events of the story around ~1177 BCE, which is when it is believed that Bronze Age Greece went past the point of no return and inevitably was headed for collapse. Essentially, reading the story as an allegory for the experience of traveling home through the remnants of a crumbling civilization.
The book you're describing does have historical depth, but due to the inclusion of a fictional story, would likely be considered literature; it sounds to me more like Jeanette Winterson's The Passion, assigned for an English critical theory class (dual English/history major), and which was used as a springboard for our professor to discuss Napoleon/French history.
Sources:
1177 BC: The Year Civilization Collapsed, Eric H. Cline
The Odyssey, Robert Fagles
Greece: The Bronze Age, Carol Thomas (Thomas is the Greek history professor from the comparison)
The Passion, Jeanette Winterson