All historical novels are not the same. Most of them simply use history as a setting, giving their characters a particular time and place for their story. Sometimes authors weave real historical figures into their plots, usually just as minor characters, or attempt to recreate real historical events. Other historical novels actually engage history, sticking as close as possible to known events, while attempting to solve lingering historical mysteries.
Melanie Benjamin did this in her 2010 novel Alice I Have Been about the strange relationship between Lewis Carroll and Alice Liddell, the girl who became the model for Alice in Wonderland. Benjamin follows known history as far as it takes her, then fills in the blanks. The result is a story is still fiction, but it is convincing enough to at least stand a chance of being true.
Lynn Shepherd does the same kind of thing in A Fatal Likeness (Delacorte Press), a novel about poet Percy Bysshe Shelley and the women in his life, especially Mary Godwin Shelley and Claire Clairmont. Shepherd's novel doesn't stray far from what is known about these people and their turbulent relationships, but it does suggest possible answers to some of the lingering mysteries, such as whether two of then women in Shelley's life really committed suicide and whether the deaths of his children might have been murder.
To try to solve these mysteries, Shepherd invents two fictional detectives, or thief-takers as they would have been called at the time. One is Charles Maddox, a contemporary of Shelley's, who does an initial investigation but then gets a little too closely involved, and the other, years later, is his nephew, also named Charles Maddox, who is hired by Mary. She and Claire remain bitter rivals years after Shelley's death, and Maddox gets thrown into the middle of their conflict. He must also try to figure what his uncle, still living, learned but has remained silent about.
This is a complicated novel, often fascinating but sometimes just dull and confusing. Those with a keen interest in Shelley's life (the question of whether Mary Shelley or Shelley himself wrote Frankenstein is also addressed here) will be ones most rewarded by reading this novel.