"On Fairy Stories" is a lecture delivered by Tolkien in which he goes over some of his thoughts on the fantasy genre, its purposes, proper audience, and value as an art form. It's full of gems, so I only have time to cover one theme of the essay. You can get it in several different collections, and I recommend this one: https://www.amazon.com/Monsters-Critics-Essays-J-R-R-Tolkien/dp/026110263X/ref=sr_1_1?ie=UTF8&qid=1479694515&sr=8-1&keywords=the+monsters+and+the+critics+and+other+essays.+j.r.r.+tolkien