A book in which Lewis reverses the usual process of criticism: instead of discussing books, and making deductions about reading from what is read, he discusses reading, and making deductions about books from the way it is read. (Or can be read -- both readings are possible for many books.)
The many -- the unliterary -- read for the events. They don't care about style or description or anything like that until it gets in the way of the events; they prefer cliche because it lets them recognize the events quickly.
He discusses some terms that he thinks not suitable for the literary readers -- the few -- like "serious" (because the literary do not read humorous or comic literature as if it were serious). He touches on some related issues of fantasy (in more than one sense), myth, and realism (in more than one sense).