Signum Crucis wrote:
The messages are the same as what we find already in the Bible, so basically they are illustrating the Bible.
I think that's a very good way of putting it. All of those authors knew something about truth, but they really weren't proposing any new truth. What an author like Lewis was really good at was taking some of the truths that were already in Sacred Scripture, putting them in terms that the general reader could understand, and applying them to our lives. A good sermon does something like this in brief, but that doesn't make the homilist a Prophet. Even the Ancient Fathers (like St. Augustine) weren't proposing new Revelation, but were instead working out the logical conclusions of what had already been revealed.
Mary Shelley was reacting against a Romantic movement that glorified (if not outright deified) the Ego, substituting not just mankind, but the Self, for God. Her novel is an illustration of what happens when man gains all knowlegde of heaven and earth, but has not Love. She expressed that truth admirably, but she was certainly not divinely inspired, and there is plenty in
Frankenstein that does not jibe with the Faith (it implicitly denies Original Sin, for one thing).
Where your instincts are very good, however, is in recognizing that good literature (indeed, all good art) proclaims to us and reminds us of truths that have new applications as time progresses. The hallmark of good writing, as Alexander Pope said, is that it renders "what oft was said, yet ne'er expressed so well."