(Spoiler alert: I'm assuming that everyone knows the plot of Jane Eyre, whether you have read the book or not. If you don't want to know what happens in Jane Eyre, you'd best stop reading this post now.)
This book is basically the story of the "madwoman in the attic" from Jane Eyre, based on hints given to her story in Jane's story.
What we know about the Madwoman: she was born in the Caribbean; she is the "infamous daughter of an infamous mother", a dipsomaniac and insane; Mr. Rochester married her due to the plotting of his father and older brother in order to bring him some wealth not normally due to a second son; following the death of his father and brother, he brought her back to England and locked her up in the tower under the supervision of Grace Poole, where she eventually burned down the house and committed suicide.
This book goes into her childhood in a post-emancipation Jamaica, her mother's poverty and second marriage, her reluctant marriage and the subsequent fall-out. An interesting concept, but I found that the promise didn't live up to my expectations. Compared with Jane Eyre, despite being set in a much more lush location (the Caribbean vs. England), I found it to be a much less rich book in terms of depth and description. I found the plot a bit difficult to follow, though that may be because the first person narrator is an alcoholic and possibly insane. I also suspect that it would make even less sense if the reader weren't familiar with the story as presented in Jane Eyre.
It does point out though, that there are multiple points of view to every story, and calls into question why the first Mrs. Rochester became insane. In Jane Eyre, it is presumed to be genetic, however in this book it is presented as a combination of childhood experiences, culture shock, repression, and alcohol.
Much as I love the book Jane Eyre and it's heroine, I never had much sympathy for Mr. Rochester - he struck me as being very selfish, vain, and condescending. And what appeals to me about Jane is her integrity and how she stayed true to her principles, no matter how much it hurt at the time. I never quite understood what Jane saw in Mr. Rochester, and his character didn't come across any more favourably in this book.
I borrowed this book from a friend who had read it from school (the same friend who recommended Pride and Prejudice and Zombies to me); and she warned me when I borrowed it that she wasn't a fan of Jane Eyre, but loved Wide Sargasso Sea. Sorry Kirsti - this is 3 out of 3 books that I disagree with you over! It did however give me a craving to re-read Jane Eyre, so I think that I will curl up with that book next.