If, which I hope is never the case with any child, we were to only ever give one book to any young person we loved (dearly, truly, with every fiber of our being!), I suppose that one book would HAVE to be "Where the Wild Things Are."
There are far too many reasons (to list here) WHY this is the case, but it is the case, nonetheless. (As far as I can see, anyway.)
Suffice it to say that, when the four children in our house were still little enough to wear their own size 6 wolf suits, this was the book that would come out when it was time to sit all together and end our own wild rumpus and enjoy another one, with Max (who we visited so often he seemed, eventually, to be another member of our family) and the wonderous Wild Things in a land that is "in and out of months and almost over a year" away.
The movie comes out...soonish? But, pleasepleaseplease, read the book first. (For a quick fix, click on the post title, at top, for a lovely rendition of the story.)
In the immortal words of Maurice Sendak: Let the wild rumpus begin.