So what can I tell you about The Mystery of Grace? It's beautiful; it's thoughtful; it's funny. It's everything you're expecting of a De Lint novel and elegant in its simplicity.
Is it the best he's ever done? Possibly not. By that I mean that the Newford novels—The Onion Girl, Spirits in the Wires and Widdershins—remain my perfered De Lint works. There's a richness and complexity to their stories. All of the perspectives and subplots weave together in a structural echo of how the stories assert the importance of social inter-connectivity. Those novels also have De Lint's rich and intricate mythos, which is missing in The Mystery of Grace.
It reminds me more of his earlier novel: The Wild Wood. Like that tale, The Mystery of Grace is more focused on one culture of myth, instead of a global synthesis. I still enjoyed De Lint's respectful handling of the Latino spiritualism and found the title character, Grace, believable. The only issue I had with characterization is that after reading the Newford novels, I expect a deeper sense of the secondary characters. As the majority of the story is told from Grace's perspective, I felt like the people populating John's world weren't as solid.
That in itself is an interesting effect—to have a novel where the living feel less real than the dead. However, it reads occasionally as a lack of depth because there just aren't as many layers to flesh out the narrative.
Perhaps best put: If you've never read a Charles De Lint novel, The Mystery of Grace is an excellent entry point. I'm uncertain how identifiable the characters would be to a young adult reader, but there's nothing in the novel—aside from mild sexuality—that a parent should question their 14–18 year old reading.
Plus, as I said, The Mystery of Grace leaves you feeling uplifted. Not in a saccharine way, either. This novel is a balm for the soul. If every act of creation is magic as he says, then De Lint remains one of our most powerful mages.