Not being given to hyperbole, let me define exactly what I mean.
I’m not talking about books I just liked a lot.
I’m not talking about books that I was exposed to so young that they didn’t change my beliefs or interests however much they shaped me.
I’m not talking about books that deepened an already nascent interest.
These are books that changed the direction of my life. The change was not a gentle, unrecognized pushing of my life along a new vector. These books were violent nudges.
Revolt on Alpha C by Robert Silverberg.
I was certainly exposed to science fictional stories before I encountered this novel in grade school. This book, though, opened my mind up to stories set in fantastical settings, here the planet of Alpha C IV. The story is a retelling of the American Revolution on an alien world. But that’s not what I remember. I remember the scene where one character explains to another that he will move differently on this world because the gravity is weaker than Earth’s. A fantasy story grounded in physics. Up to that point, I mainly liked reading history and mysteries as a kid.
It was many years before I discovered the actual genre of science fiction, though. Before high school, only one of the public or school libraries I attended to conveniently walled off science fiction in a separate section. And I hated that school and, fortunately, left it after a few months. I encountered some Arthur C. Clarke, H. G. Wells, Jules Verne, Robert A. Heinlein’s The Rolling Stones (it was decades later before I read much Heinlein, perhaps a story for another read), and Clifford Simak’s The Werewolf Principle.
The Visual Encyclopedia of Science Fiction ed. by Brian Ash
Then, tucked away with all the rest of the non-fiction in my high school library, in its Dewey Decimal System slot, was The Visual Encyclopedia of Science Fiction. Broken down by theme with plenty of pictures, it pointed the way to a lifetime of reading I still haven’t exhausted. Before I was out of my teens, I was certainly aware of a deep and vast genre waiting to be explored even if I didn’t do that very systematically. Thus, I can’t really relate to the early reading experiences of science fiction readers and writers who casually and blindly explored the field. This was discovering a Wikipedia page on science fiction before the Internet even existed.
A Scanner Darkly by Philip K. Dick
Books don’t have to change your life by their content. Sometimes it’s the act of reading them, not the words themselves, that change things. A Scanner Darkly is a fine novel, but it wasn’t the story of Bob Arctor that changed my life. It was my future wife seeing me reading it and introducing herself to me.
Unknown
The next book I don’t even know the title of. I only read its dust jacket.
I was raised in a somewhat peculiar Christian church. One of its tenets was the rejecting of evolution. For various reasons, I left the church in my 20s, but my belief in God and skepticism of evolution persisted until after I was out on college and still in my mid-20s. I had certainly read creationist attacks on evolution (old and new Earth varieties). I had also taken a lot of geology classes including historical geology. A major component of that was learning some evolutionary theory, specifically both the gradualism and punctuated-equilibrium schools. After I got out of college, I did a bit more reading on evolution.
But the final, deciding (I won’t use that trite word “epiphany” beloved by Baby Boomers) factor was a book I came across in the gift store of a science museum. It was a history of the many people who tried to reconcile the distribution of the Earth’s animals with the biblical story of all the animals emerging, after Noah’s Flood, from the Ark. (There is a lame creationist argument that the animals in the Ark were “Genesis kinds” which don’t correspond to our definition of species, but that doesn’t really explain why, for instance, you only ever found kangaroos in Australia.) It was a logical problem I hadn’t ever considered and was the final destruction of my belief in a created Earth. Upon a very slight amount of research, I think the book might have been Janet Browne’s The Secular Ark: Studies in the History of Biogeography.
MarzAat 
I hope you have read other Silverberg novels since then 😉 Some of his late 60s and early 70s novels are absolutely brilliant (I’ve never been able to stomach his early pulp stuff — Revolt is his first novel — but understand completely how important the novel is to you!)
Reading all of Silverberg is a mighty big task, and I haven’t even come close.
But I’ve bought a lot of Silverberg and am in awe of the man’s quality and quantity of output.
I’ve read a lot more of his short stories than his novels, but, of the latter, I’m fond of The Time of the Great Freeze and Dying Inside.
The Time of the Great Freeze is incredibly average — no sense of wonder, dull plot, blah.
Dying Inside is great. Loved Downward to the Earth, The World Inside, Hawksbill Station, etc.
I suspect I would like the novel version of Hawksbill Station since I liked the novella version a lot. I’ve read “A Happy Day in 2381” and liked it, but I haven’t read The World Inside which incorporates the story.
The last Silverberg novel of that period I read was the puzzling Thorns.
Thorns is the first of his “mature” novels (he has some pretty good short stories in a more social SF vein earlier… in comparison to his pulp stuff) — but yeah, Thorns sort of doesn’t work that well…
Excellent list of books!
I remember the Sense of Wonder that showered over me when I read Poul Anderson’s VAULT OF THE AGES, one of the Winston SF series. I was 10 years old when I read that shocking (to me!) book. A couple years later, I was struck by the same Sense of Wonder by Andre Norton’s STORM OVER WARLOCK. Clearly, I was an impressionable youth!
Lightning struck again in 1966 when I read DUNE. And again in 1983 with STARTIDE RISING by David Brin. Great books shape our lives.