ON TRAILS: An Exploration. Robert Moor. 341 pp. Simon and Schuster, 2016. $25. As a metaphor, the trail is well-worn. See what I mean? We can hardly help ourselves: The language of paths is idiomatically entrenched, both highly useful and ubiquitous to the point of being beyond notice. As a figure of speech, then, the trail shares a key trait with its physical counterpart: Both realize a certain kind of invisibility.