I think trying to narrow it down to 1 book, is simplistic. How do you know what is best for any one randomly chosen human?
Simply encourage them to read anything, because you do not know WHAT book might make the difference. And encourage them to go outside, to the park and read.
And encourage real books. Something where they can use their eyes to read naturally, not in the glow of some screen. We all spend too much time there already. And a real book, also stimulates the ears, and the touch, and the smell.... (And this has memory value.) And encourage them to OWN BOOKS. That way, they can write notes in the margins, or the fore leaf, or at the back. Teach them to date those notes. Someday they will look back. Maybe, even, today... they may look forwards.
E-readers and e-books are nice, but do you think they will last? No. Why? Because the machines are built to be ephemeral. As are the books. And the file formats. If anybody thinks they will still have their "e-thingie" as few as 5 years from now, think again. And what of 10 years? But a real book, can last decades, possibly centuries. You can pass them on to your friends, neighbors, kids. And you can sell them in a yard sale, to a stranger. Who just MIGHT become an acquaintance or a friend.....