There was a reason for the two books' being sold together. Vintage, the publisher, had created ten twin-packs of classic and modern novels based on a common theme, and the theme for these two was 'Lies'. In 'What Maisie knew', the lies were told to the young heroine of the piece; however, in 'Atonement', it is a young girl who tells the lie which is the catalyst for the later, tragic events. Briony Tallis, who is thirteen at the time of the first part of the novel, is a young girl who has turned to literature to deal with growing up, and this escape into fantasy is part of the cause of her behaviour. Disappointed in the play she has tried to stage to welcome her brother home from London, she loses herself in events involving her sister which she observes from a window. By the time tragedy strikes, her mind is in a condition to allow her imagination to override reason. I will say no more (mainly because you may not have read the book - or seen the film!).
One of the interesting aspects of 'Atonement' is the way McEwan creates a book inside a book. Briony later relates the events in a manuscript she sends off to a publisher, but, just as with her real life, it is impossible to have complete faith in the story she tells. At times, the line between McEwan and Briony is blurred, and you're not quite sure who is telling the story (and whether either of them can be trusted). Another nice touch is a letter Briony receives from the publisher; you get the feeling that there may have been a fair bit of McEwan in this scene of literary criticism (perhaps even a bit of tongue-in-cheek revenge for past wrongs at the hand of editors? Just my speculation).
Over the whole story, there hovers a feeling of pressure or suppression, a storm approaching, which can be felt before its arrival. From the unbearable, oppressive June heat of the first part of the novel, to the constant threat of annihilation from the air in the second, the characters appear trapped by circumstances and unable to break out and breathe freely. Cecilia, Briony's elder sister, a recent graduate from Cambridge (in the days when women could study but not receive a degree), feels caged up at her family home in the country but still strangely unable to move away and break the tie. Her mother, Emily, is overcome by blinding migraines which affect her to the point of restricting all movement or thought. Robbie Turner, a family friend and one of the principal characters, flees through the French countryside, expecting death from the sky at any moment. Later, even Briony herself experiences a form of suppression of self when she decides to do her bit for the war effort and steps outside her comfort zone. All these experiences combined give a tense flavour to the flow of the story, a flavour which enhances, rather than spoils, the enjoyment of the novel.
Of course, the major theme of the book is its title; Briony's lifelong effort to make up for her childish error is the main source of atonement, but there are other people who make mistakes (some bigger than others...) and try to erase them in some way later. Sorry; I'm not going to give any more away than that! I'll just say that there is a great twist in the book, and, not having seen the film, I wonder how the big-screen version would handle it. The cynic in me thinks that it probably isn't handled well at all...
Despite that, I would be quite interested in watching the film, to see how the story has been adapted, and, coming from a person who really can't stand spending two hours of his life in a dark room (unless I'm sleeping), that is actually a huge compliment. Reading 'Atonement' has given me a taste for McEwan's writing, and I'll certainly be looking for a few more of his novels soon (but only at a reasonable price). And that's the truth.