The biggest world-shattering book for me was the classic, Engines of Creation by K. Eric Drexler. I was just 21 and the book had a large impact on me. Nowadays though, the ideas in the book are pretty mainstream, so I don't think it would have the same effect for a millenial.
While it's overoptimistic and generally a bit all over the place, Kurzweil's The Singularity is Near might still be the most bang for the book single introduction to the "humans are made of atoms" mindset you can throw at someone who is reasonably popular science literate but hasn't had any exposure to serious transhumanism.
It's kinda like how The God Delusion might not be the most deep book on the social psychology of religion, but it's still a really good book to give to the smart teenager who was raised by fundamentalists and wants to be deprogrammed.
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.