Someone recently asked Tyler Cowen which book he would recommend to a smart person in his thirties who hasn't read much, if he could only recommend one book. This question is by no means rhetorical. I know quite a lot of smart, university educated, people who haven't read much since high school, except for textbooks.
So which book would I recommend? My initial response would be to recommend a book that is simply a pleasure to read such as The Brief Wondrous Life of Oscar Wao by Junot Diaz in the hope that, having finished it, the person will be keen on reading another book. Perhaps a better choice would be to recommend a book that is both a good read and a manifestation of what is unique about the book as a medium, such as House of Leaves by Mark Z. Danielewski, in the hope that, having finished it, the person will agree that in this day and age there is still a place for books. After all, why read a book when you can watch a movie or a documentary or listen to a podcast? If you read 20-30 pages per hour a 300-page book will take at least 10 hours to read. Consider what else you could do to nourish your mind in that time.
I also considered recommending a book that is a pleasure to read and changes the way one views the world, such as 1491. New Revelations of the Americas Before Columbus by Charles C. Mann or Thinking, fast and slow by Daniel Kahneman. Or how about Feynman's Lectures on Physics?
Leaving aside the consideration that the book should be a pleasure to read I would recommend Wittgenstein's Philosophical Investigations, but that is knowing that the person would probably put it aside after a few pages. Had I been asked this question twenty years ago I might have answered What Is Philosophy by Gilles Deleuze and Félix Guattari. It would still make a good recommendation.
As you can see, this question isn't easy to answer and I can't make up my mind. In the end I would probably recommend The Marriage of Cadmus and Harmony by Roberto Calasso. It is an excellent read and it also shows how the stories that the ancient Greeks told thousands of years ago live on to the present day in popular culture.
But perhaps I would recommend watching The Fantastic Flying Books of Mr. Morris Lessmore by William Joyce and Brandon Oldenburg, an animated allegory about the curative powers of books and writing.