In preface to my recommendations, let me say here I prefer Lewis to Tolkien (or Harry Potter for that matter). I think Lewis is just a better writer. His theological adult science fiction is an interesting (but overly long) Christian answer to David Linsay's rather curiously philosophical 'Voyage to Arcturus'.
As for 'Chronicles' I suppose some fundamentalist Christians object because they are afraid of Lewis's use of allegorical elements that are supposed to correspond with Christianity and lead you on some path to accepting the religion, but hence the danger of substitution. I don't suppose, though, it could be as dangerous as creating an alternative mythology (so isn't Bunyan safer than Milton?).
Christians see the danger, too, in some aspects of Tolkien, but I suppose its more his use of Indo-European paganism and magic that scares them the most (which would make me think good German families would be more afraid of Tolkien than Lewis, who uses such things more carefully). As to why I don't like Tolkien, it's his boring overly schematic prose (his best novels in terms of the writing have to be 'The Hobbit', and the 1/3 of 'Fellowship of the Ring').
And for real adolescent fiction, try the works of Alan Garner and William Corlett (both UK authors).
For the greatest sustained work of adult fantasy ever, it has to be the gothic fantasy, 'The Gormenghast Trilogy' by Mervyn Peake.
Fugazy
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