Reading The Catcher in the Rye, Slaughterhouse 5, and Infinite Jest within about 6 months of each other.
I was a bookish kid at school and enjoyed most of the literature we were assigned, but only really read genre fiction outside of it. I kept meaning to tackle a lot of the classics but never got around to it. After school, I got a degree in Computer Science, which took me away from them entirely and put me into career paths I was pretty miserable in.
When I was 25 I began to diversify my reading a bit. I was still sticking to genre fiction and graphic novels, but began branching out to authors I hadn't read before. Then for Easter that year my mum got me a copy of The Catcher in the Rye instead of an egg. It was the first piece of "serious" lit I'd read in years and I had a weird relationship with it where I wasn't sure I'd really enjoyed it but wanted to dig deeper. It pushed me to read some other recent classics, reading Slaughterhouse 5 and Infinite Jest soon after, both of which completely changed my conception of what fiction could do.
At the time, I'd been considering going back to uni to get a PGCE to teach Computer Science, but I decided against it because I just didn't have the passion for the subject. But reading those books I could see myself in another life teaching lit instead. It took a long time and two really miserable jobs later, but eventually I decided to get a BA in English Lit, then an MA. Now I'm doing my PhD in contemporary lit and having the greatest fucking time.
There are a lot of little things that fed into me getting to where I am now, but reading those three books in close succession basically lit the fuse.