Guide.

Books, like landscapes, leave their marks in us. Sometimes these traces are so faint as to be imperceptible – tiny shifts in the weather of the spirit that do not register on the usual instruments. Mostly, these marks are temporary: we close a book, and for the next hour or two the world seems oddly brighter at its edges; or we are moved to a kindness or a meanness that would otherwise have gone unexpressed. Certain books, though, like certain landscapes, stay with us even when we have left them, changing not just our weathers but our climates. The word landmark is from the Old English landmearc, meaning "an object in the landscape which, by its conspicuousness, serves as a guide in the direction of one’s course."

Robert Macfarlane, from Landmarks