C. S. Lewis to an inquirer, as published in The Collected Letters of C. S. Lewis, 2:644:
I myself always index a good book when I read it for the first time noting (a) Linguistic phenomena. (b) Good & bad passages. (c) Customs: meal times, social classes, what they read etc. (d) Moral ideas. All this reading, though dedicated ad Dei gloriam [to the glory of God] in the long run must not be infected by any immediate theological, ethical, or philosophic reference. Your first job is simply the reception of all this work with your imagination & emotions. Each book is to be read for the purpose the author meant it to be read for: the story as a story, the joke as a joke.
This is a nice concise summary of principles more fully unpacked in Lewis’ book An Experiment in Criticism.