The Road to Middle-Earth, Revised and Expanded Edition, by Tom Shippey. (Copyright 2003). Most literary critics have regarded Tolkien's fiction with disdain, denied that it could have any merit for an adult reader, and have consistently misunderstood and misinterpreted it, when they have bothered to read it at all. In Shippey's words, it makes “[...] many literary critics avert their eyes, get names wrong, write about things that aren't there, and miss the most obvious points of success.” Shippey's book starts with the idea that this disdain may be a reflection of the antipathy between the proponents of “Lit.” and “Lang.”, between the literary critics and teachers of literature on the one hand and the philologists, including Tolkien, on the other hand. Shippey examines how Tolkien's professional background led to and influenced the creation of The Lord of the Ring and shows the subtleties involved in that creation that are almost inevitably missed by the literary critics.
Science fiction and fantasy often regarded with disdain by literary critics, frequently even more than other “genre” fiction. I think literary critics are looking for certain specific criteria when they judge a work and are not willing to consider that their criteria may not, in fact, be universally useful in judging all works. In effect, they are blinded by their preconceived notions and miss the things that make some science fiction or fantasy good.
In practice I think that the criteria for judging science fiction and fantasy and the criteria for judging literature are overlapping sets: some criteria are shared, but there are additional criteria that must be considered when judging science fiction or fantasy, and some of the criteria for literature don't apply.