I think for me it’s Terry Pratchett’s ‘The Colour of Magic’. Not because of that book in particular, but because of it being a gateway to the Discworld as a whole.
As much as I enjoy those books, and have read almost all of them at this point, it’s more that they taught me how to be a thoughtful, empathetic person when I was a thoughtless, selfish teenager. Almost on the sly, Terry instilled values in me simply by virtue of the heroes of his stories being mostly good people who just want to have a positive impact, even if they’re flawed in different ways.
Like, Sam Vimes is undoubtedly a hero. Night Watch shows us that he strongly believes in the power of good, and that people can - and should - band together to limit the tyranny of power. But he’s also distrusting and curmudgeonly. Nanny Ogg is foul-mouthed bon viveur who places a lot of emphasis on living her best life, but she always puts her family and friends before her if she needs to. Even the Nac Mac Feegle work together for the greater good, even if, in their case, that means being able to get more drunk and fight more violently.
GNU Terry Pratchett
His Dark Materials. I was already doubting the faith I grew up with and those books helped me finally shedding all that non-sense.
