Here be Monsters: What makes a Good Book?

I read an excellent story a few days ago. It's called Young Unicorns and written by Madeleine L'engle. It has nothing whatever to do with unicorns. I'm a bit puzzled about that actually, but it doesn't matter. This book stands out from the rest of L'engle's work, as far as I'm concerned. Something that defies description makes this a good book. Something about the unusual relationships between characters - about the odd but joyful family, about the importance of music and literature to them, about the unobtrusive and yet central disability (blindness) of one of the protagonists - made the flaws in the plot almost irrelevant. The ending was too convenient and almost hurried, and yet I loved the book. That hasn't happened to me for a long time. It's tempting to be flippant and say it comes from reading too many research papers and of course I'm bound to like something escapist after that much academese, but it's not that at all. 

What I want to know is: what is it exactly that attracts me to this book so much? And if I can pin it down, how can I use it in my own writing?

Today, I was re-reading an essay by J.R.R Tolkien on the nature of faerie stories, and realized Tolkien was searching for the very same thing that I am, only he was concerned with Faerie, and not the Primary World, as he calls every day "reality". Tolkien's essay shows that he really understands that elusive quality of the genuine faerie tale - of a story that doesn't require the willing suspension of disbelief (that would imply the need for a conscious decision!), but actually creates a secondary reality, something that demands description and at the same time resists it, by nature of its Otherness. I am explaining it badly, so if you haven't already, you'd better read the essay to see where I've mucked things up. It's called "On Faerie Stories".  

Now, I want more than ever to create a true faery tale. I fear for the Owl, the Moth and the Cat - they do not quite fit into true faery tales, not on their own. But they might be minor characters in legends or motifs in ancient snatches of half-forgotten songs which provide a depth to the deliberate vagueness of faerie tale history and setting. The beauty of Faerie seems to be in its ability to elude definition. We never know the whole story: Tolkien's elaborate world was never completely finished, and I don't think it ever would be. I think he was driven to create more and more (read his essay and you'll understand more why I use that word) precisely because of the simultaneously frustrating and enticing gaps in his world. The gaps make it infinite - and we need something infinite in any good faerie story. Some assurance that there is more, that this is not all there is, that we have not yet explored the entire world. 

This makes me think of cartography. Maps were so much more exciting before everything was explored. "Here be Monsters" must have been scrawled with a kind of savage delight that cannot be found in familiar, printed place names, like "Aldergrove, BC." I certainly read ancient and modern maps with  different levels of excitement. Luckily, unlike cartography, in writing the activity of exploring a world does not make it finite (providing it is done correctly.) And Tolkien nailed it in his stories.