About fifty miles from the Inner Station, Marlow and his crew come across a small hut of reeds with a neatly stacked pile of wood out front. When Marlow explores the hut, he discovers a warning written on a flat piece of wood informing them to approach with caution. Inside the dismantled dwelling, Marlow discovers a mysterious book, which has lost its cover. The book is titled An Inquiry into some Points of Seamanship, and Marlow begins to examine the pages. Marlow mentions that there were a series of graphs, charts, diagrams, and figures inside the book that concerned sailing. Marlow also mentions that reading the material in the book makes his mind drift from the present wilderness before he notices some strange writing in the margins. Marlow refers to the writing in the margins as cipher, which adds to the mystery surrounding the book. Once Marlow arrives at the Inner Station, he meets a Russian, who is one of Kurtz's followers and the owner of the book. The Russian thanks Marlow for returning his book and mentions that the "cipher" in the margins was actually his notes written in Russian.

Marlow finds a book about seamanship which is in good condition but has notes in it in a language Marlow doesn't know. Later, he will find the book belongs to a Russian who is staying at Kurtz' station. Marlow returns the book to the Russian, who is glad to have it back.This is the same man Marlow had heard the station manager and his uncle discuss killing earlier in the novel.