On the most obvious level, this is a story about the effect of British colonialism on the life of an African village. It's also a portayal of the brutality and beauty of that way of life; and most impressively, a successfully sympathetic portrayal of an angry, unlikeable man.
Okankwo is an ambitious and industrious man, driven by his feeling of shame about his lazy, unsuccessful father. When the book opens he's rich, respected, and the head of a thriving family, an important member of his community, a wealthy and proud village. When the book ends - well, to avoid spoiling the ending too much, I'll just say that he's not and neither is his community. The novel is the story of his descent from the peak of his success. He's not a very likeable man. He beats his wives and children. He is proud and cold. But Achebe does such a good job explaining the motivations behind his repulsive actions - his childhood shame, his worries that his son will be weak like his father, his secret fear of being thought cowardly and his secret love for his children - that it's impossible to do other than forgive him. Okankwo is a hard man, but he is the product of a hard life. Achebe dwells on the beauty of village life - moonlight plays, music, and rituals; but he doesn't turn away from the harsher aspects. Twins are 'thrown away' - abandoned in the forest to die. And the gods of Umuofia are bloodthirsty.
Okankwo commits a terrible action because of his fear, and his life starts to fall apart from that moment. The theme of colonialism isn't introduced until near the end of the story and the decay of the community is merely the cap to his spiritual losses.
The language is fantastic - simple, melodic, rythmic, full of character. The reader, Peter Francis James, was a joy to listen to. I highly recommend both the book and this particular audio recording.