Margaret Atwood was right, this book did knock my socks off, to the point that sometimes it got uncomfortable to read. I find it works both as a thrilling dystopian novel and a thought-provoking feminist text, though the two are intertwined perfectly. 
 Really short summary: The novel's set in a distant future, around two thousand years from now. Our age is known as the Cataclysm Era because it ended with a world-wide devastating war. New civilizations arose, and at the point in which the novel takes place they're living in a society much like ours. The only difference is that, after the Cataclysm, women were all born with a skein, an internal organ that produces a current of energy that can be used to hurt, control or even cure--aka, "the power". Because this made women more powerful, all societies recorded by history in this new future are Matriarchal ones. The twist, the fun thing though, is that the only glimpse we see of this future are the emails exchanged, at the beginning and ending, by the author and editor of the "historical text" that is the body of the novel. The novel itself is what the author (a man) speculates might have preceded the Cataclysm. In other words, the novel takes place in our time, our present, our Patriarchal world, at the moment women started to be born with that internal organ and power started to shift, from men, to women. 
 To me, this novel had three major themes going on, all of them very interesting: (this will get a bit spoilery, though I'll refrain from giving too much away) 
 1) There is the idea that power is a corrupting force--quite an old but still relevant concept. That it makes people do horrible things just because "they can", be they men or women. That is, that if women suddenly became more strong and powerful than men, they would do as terrible things as men have done, and that a Matriarchal society would be (or be able to be) just as unjust as a Patriarchal one. This is a discouraging thought, but the novel is hopeful that there will always be exceptions to this rule (view spoiler)[our sort-of-heroes Tunde and Roxy (hide spoiler)]. 
 2) The point that I find more interesting, and I think Naomi Alderman was trying to make, though, is not that women are not morally superior than men and as capable of horror as them. The main point is that all these horrific things women do to men thorough the novel--which as a reader you are repulsed by--are actually being done by men to women RIGHT NOW. But to see it reversed is eye-opening and provoking--and we are witness to all sort of injustices, of escalating cruelty: men being afraid to walk alone on the streets, avoiding eye-contact with the women they cross paths with; women attributing the work of men to themselves; guerilla women mutilating men's genitals; women raping men and filming it just for the fun. The book shows you these horrible scenes and part of you, horrified, goes "people would not let things like that happen, certainly someone would interfere before we got to that point", and then you go: Oh, right, this is actually happening in the real world. But it is not all so dim, there is also a lot of humour in it, especially in the aforementioned emails. The male-author constantly apologising and thanking the female-editor for taking the time to read his novel, the editor, ever-condescending, warning him that this idea of male-soldiers is very sexual and can be seen as "cheap porn": certainly a group of men in uniforms will make all readers giggle, and is not very realistic. The female news-anchor patronising and putting her hand over her male co-anchor knee was also a nice fun touch. There are thousand more examples (some funny, some terrifying) but I'm falling asleep, so I'll write them tomorrow. I think the icing of the cake in that regard is actually the last twist, the last sentence of the (real) book. 
 3) The fact that our era ends with a nuclear war is quite a recurrent one in sci-fi and dystopic fiction, and that's because we can already see some signs of it in real life--it's not a stretch of the imaginations to imagine a thing like that happening, though Alderman's war factions are quite a novel idea (men vs women). By emphasizing how silly and terrible are the causes of the war, I think the novel points towards the need for reunification, to work together not only as men and women but as people of many races and countries. 
 Though I found these to be the main takeaways of the novel, you could also focus on many other things, and the novel presents many ideas that could be discussed over: from History and how it is perceived and altered through time (some of these bits were hysterical), to the fact that Man Activists will always be THE WORST in all kinds of civilization. 
 (I also apologise for typos, as I said, I'll edit this tomorrow: SORRY)