As for Veganism, what convinced me was the images and sounds in Meat your Meat, followed by hearing Gene Bauer speak, followed by coming across this very website of vegan athletes 7 years ago. The video was more powerful than all the books in the world could have been...
For non-vegan influential books, I may have posted on this thread years ago, but here is my list now:
Marcus Aurelius' MeditationsBasically a journal of brief philosophical musings.
Richard Dawkins, Christopher Hitchens, and Sam Harris' books had a lasting impact.
For fiction, William Gibson, Gary Shteyngart, Jonathan Ames, to name a few....