What book has as a central theme of boat ride journey up river in Africa?

The book you're referring to is likely "Heart of Darkness" by Joseph Conrad. The novel is set in the Congo Free State in the late 19th century and tells the story of Charles Marlow, a European trader, and his journey up the Congo River to find Kurtz, a mysterious ivory trader. The novel explores themes of colonialism, imperialism, and the human condition.