In 1937, C.S. Lewis wrote a review of Tolkien's newly-published The Hobbit

Excerpts of that review, published in the Times Literary Supplement:

The publishers claim that The Hobbit, though very unlike Alice, resembles it in being the work of a professor at play. A more important truth is that both belong to a very small class of books which have nothing in common save that each admits us to a world of its own—a world that seems to have been going on long before we stumbled into it but which, once found by the right reader, becomes indispensable to him. Its place is with Alice, Flatland, Phantastes, The Wind in the Willows...

To define the world of The Hobbit is, of course, impossible, because it is new. You cannot anticipate it before you go there, as you cannot forget it once you have gone... Though all is marvellous, nothing is arbitrary: all the inhabitants of Wilderland seem to have the same unquestionable right to their existence as those of our own world, though the fortunate child who meets them will have no notion—and his unlearned elders not much more—of the deep sources in our blood and tradition from which they spring.

For it must be understood that this is a children’s book only in the sense that the first of many readings can be undertaken in the nursery. Alice is read gravely by children and with laughter by grown ups; The Hobbit, on the other hand, will be funnier to its youngest readers, and only years later, at a tenth or a twentieth reading, will they begin to realise what deft scholarship and profound reflection have gone to make everything in it so ripe, so friendly, and in its own way so true. Prediction is dangerous: but The Hobbit may well prove a classic.

You can read his review in toto at The Paris Review.