The book that most shaped me as a person was Philip Pullman’s The Golden Compass (and the other two books in the trilogy). (I read the first book in third grade, so I consider that a ‘child’.)
It was the first time the thought ever crossed my mind to question religion, which is pretty much where I’m at now…questioning. (I like how Pullman explains religion in those books, too, and how some of his questions are questions I’m wrestling with because they’re good questions.)
I also attribute some of my fascination with atomic physics/string theory/planes of existence/etc to those books, especially The Subtle Knife, (although TAS also deals with it).
I also like that Pullman explains/gives a great reason for certain things that occur at the end of The Amber Spyglass that other authors didn’t. After thinking and thinking and thinking about this, I realised that I’m one of those people who needs a reason for pretty much everything I do, and when other people ask me to do things, I want need a reason. Otherwise I dig my heels in. :)
The different societies and cultures explained in the books were also inspirational to me, as an anthropology student (this is, however, something I’m only now realising).
Also, Pullman is/was one of my inspirations to become a writer (Neil Gaiman is another). Like these two, if I end up writing, I want to write something with substance, that is hard to categorize, has lots of deep thought, and can change lives (and is fantasy, of course).
Oh, and, the (original, blue-and-yellow) Nancy Drew books were also huge influences…I’ve always loved mysteries, puzzles, and crime procedurals. (I know you said only one book, @girlofscience, but I read the Nancy Drew books a lot earlier than Pullman’s trilogy. So arguably they had more influence. sorry)