I remember reading A Brief History of Time when I was 13 years old. Before then I dined on fantasy fare involving swords, magic, wizards, dragons, and the like. So it fit right in, but was cooler, because it was real. It blew my young mind away like no other book has since, or could ever hope to again probably. Besides the quality of the content, it came into a certain time in my life and shattered a paradigm. Pretty much cemented it on a pedestal of influence.
That is a great way to explain what it feels to read something that transforms your entire world view. I had that experience with Guns, Germs and Steel, The Singularity Is Near, Maps of Time and The Ancestor's Tale.
I have to read GG&S. I accidentally bought my father a copy when he already had one. I can grab it any time. The sheer number of times I've seen that book referenced demands it. Funny how a book being in hard copy form now makes it less likely I will read it...