It’s by Jean Rhys: it’s a feminist, post-colonial take on Jane Eyre from Bertha Rochester’s POV, detailing her past in the West Indies and just how she came to be locked in that attic. It’s a while since I’ve read it - used to be a favourite on high school reading lists - but it’s not that long, and probably worth checcking out.
But she forgot to say “thank you” and “please” to him. She deserved everything she got.
As was pointed out, the book is actually WIDE Sargasso Sea. Just to amplify on Scissorjack’s post (possibly unnecessarily): One of the major plot points of Jane Eyre was Mr. Rochester’s inability to marry Jane because he was already married. His first wife was insane, and he kept her shut away in the attic. Jane and Mr. Rochester were only to able to find happiness at the end of Jane Eyre because the crazy first Mrs. Rochester killed herself (and in the process burned down most of the house, Thornfield Hall).
In Jane Eyre, the first Mrs. Rochester is essentially a plot device. Her history is not given and she is never developed as a character. Indeed, the “mystery” elements of Jane Eyre stem from not knowing who Mrs. Rochester is, what her condition is, or indeed even that she exists.
Wide Sargasso Sea tells the story of the first Mrs. Rochester. It takes a character who is two-dimentional and gives her a history and a context. It also makes the reader reconsider Jane Eyre, because once you consider Mrs. Rochester as a human being, Mr. Rochester’s behavior comes much less excusable and it is difficult to view him as the romantic hero. It’s a pretty good book.
…Ha! You discovered my inspiration! 
Thanks for the comments…
Huh. Sounds a lot like Wicked or Rosencrantz and Guildenstern.