First of all: This whole thing about "not telling" is just a marketing gag for me and completely unnecessary. So I will write this review with references to the plot but obviously trying not to spoil it, as I would with any review about any novel. The novel is excellent, and having a basic idea about it does not take away from the quality of story, language, and narrative.
Now:
The story is told from two perspectives: that of Little Bee, a girl from Nigeria who as the novel sets in is about to leave a British Immigration detention center, and a successful middle class white English woman. The lives of these two women were joined in a most horrid incident two years earlier. This incident is finally related towards the middle of the novel. The English woman is forced to deal with its consequences on her husband and her marriage. For the African girl it means she has to be on the run, both in Nigeria, and in the UK to where she fled but where she has not been granted asylum.
The life of refugees and their reasons for fleeing form one part of the book. The other (closely related) aspect it artfully illustrates are the human cost of the clash between the third and first worlders.
The main characters are presented skilfully. Their inner conflict is plausible and touching. The character of Charlie a.k.a. Batman, the English woman's 4-year-old, is endearing and truly works as the "emotional center" the author wanted to create (interview with Cleave).
My only criticism would be with some of the dialogues, which sometimes seem to stretch beyond the necessary.
Of all the good things I can say about this book (Narrative, voice, language, characters), what I find most important is that it tells a story that needs to be told. In times when Europe braces itself against immigrants and is getting ever more unwilling to grant asylum, when our oil companies continue to exploit Nigerian oil at any human cost, we need stories like this to make us grasp emotionally what we read about in the papers as we eat our breakfast cereal.