I actually have not read extensively for fun since entering college because I have spent much more time reading textbooks. The books I have read as an adult did not stick with me the way the books I read as a teen did, for the most part. I am going to list the books that made a difference to me over my whole lifetime together. However, be aware, that if you want to have an in depth discussion of books, likely I'm going to fall a bit flat. Even though at one time in my life, I spent every moment of downtime reading.
Enders Game (and series)
Blue Adept (and Apprentice Adept series)
Incarnations of Immortality
Memory of Earth (and Ships of Earth series)
Snow Crash
The Diamond Age
Stranger in a Strange Land
Heralds of Valdemar series
Mistborn (though I have not read the whole extended series)
The Hobbit
I think it is fair to say The Lord of the Rings series made an impression, though I have actually not made it through the whole thing. I got seriously stuck in the battle in book 2, twice.
The Dark is Rising series
Chronicles of Narnia
The Giver
Nightfall
Isaac Asimov's Robot series
Foundation series
Maybe: Tale of Two Cities, The Shipping News, The Invisible Man, Zen and the Art of Motorcycle Maintenance, The Yellow Wallpaper, Cask of Amantillado
Definitely: Basically all of my best friend (after graduating) and committee member and colleague Steven R Brandt (Steve)'s books
Unfortunately: Harry Potter, although I really do not like what JK Rowling has to say
Star Trek: My Enemy, My Ally-- although I am not sure I am remembering which book correctly, since the book I am thinking of features Kirk, Spock, and McCoy-- I am literally trying to cite a book that influenced me, not referring to someone as my enemy/ally