WARNING: A PLOT SPOILER IS INCLUDED IN THE FIRST PARAGRAPH
This book was written as a prequel to Jane Eyre (JE). It focuses on Rochester's first wife, JE's `madwoman in the attic'. In chapter 27 of JE we are given a brief back history of this woman and of how Rochester came to marry her, but this is recounted by Rochester himself: we never get to hear from her, despite her importance in the plot of JE. By contrast, in WSS Jean Rhys makes her the centre of the story as Antoinette Cosway; the name `Bertha' by which she is known in JE is foisted on her, against her will, by Rochester; this is one of several ways in which Rochester appears in WSS as an oppressive and bullying man. After she and Rochester marry she develops some disturbing behaviour symptoms which eventually turn her into JE's `madwoman', but WSS implies that this behaviour is not (as Rochester claims in JE) hereditary but instead is the result of his poor behaviour towards her. The story in WSS takes us through her life from a young girl to her eventual suicide; the bare details of the suicide are recounted in chapter 36 of JE, but WSS provides an explanation based on Antoinette's gathering despair at her treatment and her hopeless predicament.
Writing a prequel or sequel to any famous and widely admired book is bound to annoy some people who fear that the original work is being exploited, or that its themes and characters are being distorted. There is evidence of this in some of the readers' reviews of WSS. The most frequent complaint is from reviewers who object to WSS on the grounds that it turns Rochester from (what they see as) JE's romantic hero into a villain. In my view this criticism is based on a misreading of JE, which surely presents him not as a hero but as a flawed character: for example, his willingness to involve Jane in what would have been a bigamous relationship is not romantic but instead is deeply selfish, since in the context of the times this risked a huge scandal which would have tainted Jane as well as destroying Rochester. So, if he comes across as selfish in WSS I would say that Rhys is being true, not false, to JE.
More generally, I felt that WSS is true to, and in some ways enhances, JE's themes and ideas. JE was arguably the first ever novel in which the central female character is more impressive than her male counterpart (Jane is better educated, more intelligent, and in every sense a more moral person than Edward Rochester) and in which the female character's life objectives are taken as legitimate in their own right rather than subservient to those of a male character. For these reasons it is a landmark in the history of feminist literature. WSS in no way distorts this central theme: on the contrary, it adds to it by presenting a somewhat similar story (Antoinette is likewise superior to Rochester) albeit in a more complex context in which the central male-female relationship is complicated by cultural clashes between natives and colonialists. So, I certainly didn't feel that WSS is in any sense exploiting or distorting JE; on the contrary, it's entirely consistent with JE's approach. Moreover, in some ways it made me appreciate JE even more than I already do, by emphasising the timelessness of its themes.
I was already very familiar with JE before I read WSS, and I find it difficult to imagine how I would view WSS if I had read it without ever having read JE. Does it stand up as an outstanding novel in its own right, independent of its distinguished ancestor? I suspect not, partly because it doesn't provide the reader with enough feel for Rochester's character and motivations and in this sense is a little unbalanced as a stand-alone novel. I can see why Rhys didn't expand her treatment of Rochester: she presumably assumed that most readers would already be familiar with JE and would therefore simply transfer their acquired knowledge of the character to WSS. Of course this is an inevitable dilemma for the author of a prequel/sequel, and I don't want to criticise Rhys: it's impossible to write such a book in a way that is wholly satisfactory regardless of whether or not the reader is familiar with its ancestor, and I am sure she made the right decision in aiming the book mainly at those already familiar with JE.
As a big fan of JE (and indeed of all the Brontė novels) I would certainly recommend this book to anyone who has already read JE. To someone who has not, though, I am not sure that I would recommend reading it as a stand-alone novel.