I like the characters; they're captivating and are what made me want to finish the book. I like the fact that they play imaginary-world games together but do not fall into the cliche of visiting their own imaginary world. I like the fact that the fantasy element is elusive and possibly imaginary, manifesting itself in the form of what would be awfully eerie coincidences. I like the fact that the secret quest takes the heroes to an obscure town in eastern Ohio that I've actually visited. I like the it's-assumed-you-know-them references to earlier fantasy that the characters have read (especially Lloyd Alexander, and also Tolkien). I like that this is a present-day book in which the heroes are able to get away with doing things that are illegal (stealing bicycles and a sailboat), immoral (tearing pages out of a library book), and fattening (eating donuts for breakfast).
Here's another thing I particularly liked. The protagonists are three pre-teens whose friendship had been forged by improvising stories taking place in their shared imaginary world. But now the youngest of them fears that the other two are outgrowing it.
"I thought that ... we'd have something that no one else had - an experience that would keep us together. I can see you changing." She turned to Zach. "You're going to be one of those guys who hangs out with their teammates and dates cheerleaders and doesn't remember what it was like to make up stuff. And you--" She whirled on Alice. "You're going to be too busy thinking about boys and trying out for school plays and whatever to remember. It's like you're both forgetting everything. You're forgetting who you are. ... We had a story, and our story was important. And I hate that both of you can just walk away and take part of my story with you and not even care. I hate that you can do what you're supposed to do and I can't. I hate that you're going to leave me behind. I hate that everyone calls it growing up, but it seems like dying. It feels like each of you is being possessed and I'm next."And that is what happened to Susan Pevensie in the Chronicles of Narnia. Is it clear now? Can we get out of our thick heads the ridiculous notion that Lewis damned her to Hell because she'd become sexually mature? That's not what this was about in Lewis, and not in Black either.