I'll make two exceptions to that generalization, at least as far as Narnia books are concerned. One is Paul Ford's Companion to Narnia, which is very informative and a fun read in itself (the author was a classmate of mine at Fuller in the 'seventies); and the other is Walter Hooper's Past Watchful Dragons (1979), now apparently out of print (and drawing outrageously high prices on the used-book sites). Hooper's book should have been read by the media types who talked loosely (and complainingly) about the "Christian allegory" in the Narnia movie. He quotes Lewis deftly on the subjects of allegory and symbolism, explaining the differences, and adds some value for Lewis-readers by incorporating some of Lewis's unpublished material (including a longish draft of what was to become The Magician's Nephew).
Hooper's title comes from something Lewis wrote about his purposes in writing the Chronicles.
I thought I saw how stories of this kind could steal past certain inhibitions which had paralyzed much of my own religion in childhood. Why did one find it so hard to feel as one was told one ought to feel about God or about the sufferings of Christ?Unfortunately, for Christians and non-Christians both, Narnia is itself rapidly acquiring its own "Sunday school associations" (something that Hooper saw might happen and warned against) and the media dragons were out in full force, determining that no "religious Right" propaganda should pass unscathed. Hopefully there are more Christian artists out there who can make new stories that are dragon-resistant.
I thought the chief reason was that one was told one ought to. An obligation to feel can freeze feelings. And reverence itself did harm. The whole subject was associated with lowered voices, almost as if it were something medical. But supposing that by casting all these things into an imaginary world, stripping them of their stained-glass and Sunday school associations, one could make them for the first time appear in their real potency? Could one not thus steal past those watchful dragons? I thought one could.