The answer to this prompt depends on how far back you go. When I was small I would say Harold and the Purple Crayon, in which Harold uses his purple crayon to draw himself having all kinds of adventures. As a kid who liked drawing, it gave me — permission? license? — to insert myself into whatever I drew. That in turn made me draw much more speculatively, rather than just copying cartoons and such.
If we’re not going that far back, then I would say the Hobbit/Lord of the Rings trilogy, which I read at about 12 years of age. Having mostly paid attention to sci-fi until then, it was an entirely new perspective on what deserved my attention. Prior to that, if it wasn’t about zooming spaceships and/or robots, it wouldn’t catch my eye. It was a very dense text at that age, but I have re-read it many times since and discovered new ways of seeing the story and characters each time.
Maybe it deserves another re-read, come to think of it.