When I was about a young kid, before high school I know, I was obsessed by Greek mythology. My dad had a book on his shelf called Paideia, so I picked it up. It was about the Greek ideal of education as manifested in philosophers like Plato, etc... I read this book and I swear I actually understood it, although I might have been 12. The reason why I can claim I understood it is because I remember following the argument with some excitement and not skipping over anything. It took me a while to read and it taught me something about how an intellectual proceeds.
I think this book, more than any other, made me an intellectual. The Greek ideal of education was based on the inculcation of a concept called "arete," or virtue. This concept was not a stable one, but seemed to shift shape from one philosopher to the next, sometime radically, along with the meaning of paideia itself. This was fascinating to me, because I expected the Greeks to be saying one unitary thing, not to have contradictory ideas and arguing with each other. Education could be so many different things, and I was myself educated by this book, even though I have rarely thought of it since. Of course, I don't remember specific points in the argument any more. (I should re-read it now. If anyone wants to get it for me for X-Mas I will give you my address.)
Paideia, of course, is another word for Bildung, or formation. The idea was how to mold the subjectivity of the student. Jaeger was a German philologist writing in the 30s, although I had no way when I first read this book to understand this context. I saw Jaeger just as a guy looking at the texts and explaining what they were saying, not as part of some grand hermeneutical tradition of German classicism.
Anyway, my point is that what we read shapes us, or deforms us. That's what literature is for, to shape and educate, to "kick us on the ass with its transformative power" in my previous formulation. If that is not happening we might as well forget it. Of course, I don't really distinguish between poetry and philosophy, because both can have this power.
This might explain why I became critic and not writer. I need that level of engagement with abstract concepts to be happy. Writers are smart in other ways, but not always in this particular way.
I was thinking about this last night: what books shaped me most as a scholar? I would have to say that this book was the first, and hence most profound influence on me, though I guess if I was reading it in the first place I was already what it was asking me to be.