What book does Marlow find in the reed hut in the jungle? How does he feel when he puts the book away? Why?

As he steams up the Congo river toward the Inner Station,
Marlow finds a navigation manual in the reed hit in the jungle. Inside the navigation
manual are strange notations in the margin that Marlow cannot read. We learn later that
the book belongs to a Russian who works for Kurtz.

The
navigation manual is a symbol of Marlow's feelings as he journeys up the river. In many
ways the manual should be of great use to him and he should be very happy to find it.
The strange notations (probably written in Russian), however, cause him to further
wonder what he has gotten himself into and what possible people and events he might
encounter when he reaches the Inner Station. The book, and the journey itself, are
beginning to make Marlow very nervous.