I have not read the whole series. I think I stopped at #4, not because it had jumped the shark or anything, I just usually get bored with reading the same authors after a while, especially when they aren't stylistically diverse. I haven't read that Hitch book either. Do you think you could summarize what he said?
I did mean to imply that everyone IS formed that way, whether consciously or unconsciously.
One statement most every creative writing teacher gets (I've heard) is from at least one student who says that they don't want to read poetry for fear of being influenced and losing their originality; the assumption being they haven't already been "formed" by what they know (or think they know) about poetry. Yet, If they aren't well-read, they can't really know what's original and what's not to begin with. So, in a sense, they've already been formed by influences; but because the influences are unconscious, and because they're so ignorant (of what poetry is out there), it deludes the students into thinking they haven't been influenced at all. I think that's one example of "unconscious influence."
However, I also think that it can be, at least partially, conscious. Keats picked over Shakespeare's sonnets with a fine-toothed comb, writing notes in the margins about what he liked and didn't, and out of that "study" he formed much of his own aesthetic ideals. So I think that's an example of a very conscious kind of influence. In my own writing I often find that when I'm looking for an image, a metaphor, or some stylistic/formal device I find myself consciously reaching for things I've read; or I've found myself consciously "imitating" poets/poems I love in hopes that maybe some of their creative process will rub off on me, that I'll learn from trying to employ it myself.
So I might say that there's always some unconscious influence which is based on one's level of experience with their art; yet, with good artists especially, there's almost always a conscious side to influence as well. I'm probably not as "careful" as you might recommend. I tend to want to try everything at least once. Sometimes I quickly learn that, for whatever reason, I'm not wired to write in a particular way. I attempted (a few times) writing a satirical piece via couplets ala Pope, or very simple lyrics ala Burns, and found that I just couldn't.