I'm sort of unsure whether to put four or five stars for this. I wish I could give four and a half star. It was indeed terribly good.
It is a mix of all I love in a book; a magical world and a child who enters it, growing strong and conquering its own fears and the dangers the world presents; and spun into all this, metafictionality, where Connolly turns familiar fairy tales into something new. I love the subvertisy of it.
Also, all the characters are really well written, with distinct personalities and individual voices. The magical world is wonderfully described, and this goes for the reality in David's actual world as well. The historical events alluded to gives it depth.
Connolly is also a master of hinting and subtlety. Many of the things talked about in the book might be too obscure to be understood by a child (and the protagonist in the book does puzzle at some of them), whereas they give the story a richer, fuller background for an adult. I love that.
Related to the obscure hints are the obscurity and ambiguity of the story itself. At times I found myself uncertain of what was really happening, Connolly providing an oppportunity for several interpretations - even though he in his appendix to the book clearly states what his view of the reality in the book is (which differs from my own) - but that did not lessen my enjoyment of the story in least.
All in all a wonderful book, tethering on receiving full five stars.