William Empson, "Seven Types of Ambiguity", Pelican 1973, first published 1930.
Still a fascinating work that is fun to read in bits, though perhaps a little indigestible if swallowed whole. For example, he points out that "it" [poetic meaning, ie what is described by poetry]: "In a sense cannot it cannot be explained in language, because to a person who does not understand it any statement of it is as difficult as the original one, while to a person who does understand it a statement of it has no meaning because no purpose" [p 22]. Of course, Empson was constantly in danger of being thought to mean that explanation is to provide a substitution for its object rather than a way of helping by alternative imagination of the means to knowing what a poetic description is up to. Of course, Empson was surely well aware of the dangers of approaching poetic meaning by making prosaic some routes through the normal maze that is the natural ambiguity of ordinary language and the explosion of this modest maze of ordinary language into the vasts and ramifieds of poetry, and of 'finding' poetic meaning through collapsing the language that carries it (a kind of Wittgensteinian throwing away of the ladder used to climb to the point, once having got to it). Empson can be seen at work, among many others, in the superbly imaginative scholarship of Christopher Ricks.