The written word symbolizes the power of Europeans, which Achebe tells us outright. We see that Isaac Okonkwo embraces it early on:
Mr. Okonkwo believed utterly and completely in the things of the white man. And the symbol of the white man's power was the written word, or better still, the printed word… (13.24).
It isn't just the two Okonkwos who recognize the power of the written word. The Umuofia Progressive Union is made up of poorly educated men who recognized the power of knowing "book," and who saved their pennies scrupulously to send one of their "sons" to England to go to college. They knew that having an educated kinsmen would give them access to the upper echelons of power within Nigeria.
Both Obi Okonkwo and his father Isaac embrace the written word as part of their identity. As a result, both seem to identify with European values and culture, at the expense of their own people. Obi becomes engaged in a life-long pursuit of European power by relying on his literacy. He soon realizes, however, that is not enough. Disengaged from the traditions of his people, he nevertheless gets caught up in the corrupt African power system that involves bribery. However, and perhaps because of his life-long pursuit of the written word, he lacks the street smarts to be effective in his corruption. In an ironic turn of events, he is soon caught and sentenced in a European court system, by European values, which he had adopted by embracing the written word.