The answer to that question depends upon who's answering.
For me, a good book takes me somewhere unfamiliar and makes me believe it. Puts me in another's shoes so convincingly that I forget myself. Thrills me with language and imagery and meaning. Challenges something about the way I see or think about the world. Rewards me at the end by leaving me thinking for hours, days, weeks, years. Invites me to re-read it. Makes me want to own it, so that passing the shelf where it waits, the book beckons and reminds me of its multiple pleasures.
Based on my criteria, Brian Selznick's The Invention of Hugo Cabret, a work of children's literature and winner of the prestigious Caldecott Award in 2008, is a good book. A complicated story combining silent film and images and text, Hugo Cabret touches on such subjects as abandonment, being orphaned, art and mechanics, fantasy and reality. Written for children, it dumbs nothing down and in the process makes demands and rewards the reader who sticks with meeting them.
Almost as engaging as the novel is Brian Selznick's website. Visit it, and then read the book. I have no doubt that you'll agree it's a good one.