A speaker looks at an old, cluttered pile of books and sees something much older: the remains of a grove of trees. Paper comes from wood, so the books are like strata, the layers of rock or sediment that settle over time. The trees were buried, and the books are what remain of them.
The second stanza turns the idea toward the speaker's own life. These books are "telling me how to fall / and in which hard bed." Falling suggests dying, or perhaps simply giving way to age, and the "hard bed" could be a grave or the ground. The books seem to offer a lesson in how to end, the way fallen trees do.
The mood is quiet, thoughtful, and a little grim, but not panicked. The speaker seems to accept mortality, finding a strange companionship in the books. Reading and decay feel like part of the same natural cycle, and the title's "bookworm" adds a wry touch, since worms also work on what has fallen.