In Wide Sargasso Sea, Jean Rhys tells Bertha's story and her descent into madness.
Her name was Antoinette and she was the Creole daughter of a slave owning family in 19th century Jamaica. The family fell into financial and social ruin when the British Empire outlawed slavery. The local freed slaves hated them enough that they eventually burned down their home and killed Antoinette's brother. Her mother married a wealthy Englishman, who killed himself, driving her mother to madness. Antoinette was raised by nuns at a convent and eventually an Englishman married her for her inheritance. But the unnamed Englishman (presumably Mr. Rochester) was unloving and believed rumors that he heard about Antoinette and her family. He became distant and unfaithful and even began calling her "Bertha" instead of her real name over Antoinette's objections.
The book humanizes Bertha / Antoinette, who is no longer the monstrous lunatic of Bronte's novel. Her troubled mind is in part due to heredity and in part to her traumatic upbringing and unhappy marriage. Her marriage to Rochester resulted in the loss of her freedom, which is analogous to the slavery inflicted by her own ancestors. The novel explores conflicts between races and genders and classes and forces the reader to think about the choices we make in life. Although Antoinette looks white, she is part black and not fully accepted by either race. Her marriage to Rochester is, in part, an effort to escape her situation.
Sargasso is probably the most interesting examples of fan fiction I've read. The narration suddenly shifts from Antoinette to Rochester without explanation, which was a bit jarring and confusing. But a quick re-read of the opening pages of Section 2 clarified my perspective.
It isn't necessary to read Jane Eyre before this novel, but it helps one appreciate where the character is going. If you have not yet read Bronte's classic novel, I recommend reading a synopsis before diving into this one. The book does stand on its own; but is better as a prequel and explanation of Bronte's novel.