Drood by Dan Simmons. Although most of his earlier books are horror, sci-fi, or detective noir, he also has a few books where he mixes fact with fiction. Fact is very well researched and fiction fits nicely into his narrative.
In the case of Drood, he looks into the real life relationship between Wilkie Collins and Charles Dickens and the decadent but fascinating society that they moved in. That plus their relationship with the real women in their lives were all nicely developed. Then the addition of the fictional antagonist, Drood, adds the suspense element and a somewhat supernatural aspect to the book that keeps the narrative flowing and interesting. The antagonist is very roughly based on the main character of Dickens unfinished book, The Mystery of Edwin Drood.
His description of late 1800's London is extremely realistic and disgusting. It was well researched and left me wondering how people could live in those conditions.
Just finished 2 other fact/fiction books by Simmons:
The Fifth Heart where Henry James meets Sherlock Holmes (who, ironically, wonders if he is real or a construct of an author's immagination) as they attempt to stop an assassination of Pres. Cleveland during the world exhibition (The White City) in Chicago.
Abominable where the early days of climbing Mt. Everest and the death of Mallory and Irvine on that mountain. The bulk of the story, however, has to do with some very real aspects of pre-WWII activities of the soon-to-be Nazi party.
Simmons is one of those rare, imo, authors whose prose flows so smoothly that he can suck you into almost any topic and make it believable and interesting.