Okay at the risk of being offensive, all the comments before mine are made by overly intellectual fools. I'm not usually so insulting--but as a father of two, if my 3 year old and 1.5 year old could only read one book, it would have to be "Oh, the places you'll go" by Dr. Suess. Sorry to stomp on Das Kapital and its ilk, but if you get only one book, I can't imagine a better first message to convey than the endless p…
But I think the question actually meant: "for actual real life people here on HN, who have all probably read some books already, what extra book would you suggest reading?"
For us adults, there are probably books that would have more value than Dr Zeuss, excellent though it may be.