Many (most?) books I've read have had an influence on me. The one that had the biggest impact on my world view, both at the time I read it and in the years that followed, would probably be
Flowers for Algernon. I actually read the short(er) story, as it appeared in Fantasy and Science Fiction, before re-reading it as a paperback novel. I think the shorter story had the greater impact, but both versions are powerful, IMHO.
I first read it in my early teens, in the 1960s. At the time I was one of those kids who breezed through school with Bs even though I never cracked a book or handed in homework. Reading about a developmentally disabled man, struggling to cope with the world around him, realistically written in the first person, really shook me.
The ending of the book was crushing.
In the past few years I've had the opportunity to meet many disabled people, and I've never forgotten that book.