In the order that I remember reading them:
Grimm's Fairy Tales. I remember this old paperback because several of the stories were older versions and so did not align with the Disney movies I'd seen. Ashputtel, Mother Holle (which was also the first place I read the word 'slut'), and the Juniper Tree all nestled side by side, with witches and talking animals galore.
Greek myths in a set of encyclopedias . My sister and I are apparently unusual in that we were raised in a truly secular household - prior to my parents' divorce, I think we went to church with my Grandmother exactly once at Thanksgiving. In our bedroom we had a set of ancient encyclopedias, and I read and re-read the myths about a million times. 'God' was a vague concept; the gods were people.
Pet Semetary by Stephen King. I read this in grade four, and it crashed the concept of death into my brain. Never did get over it.
Some Fucking Book. I've been sitting here googling for twenty minutes now trying to remember what the everloving hell this book was, but no dice. I DO remember for grade eight or nine we read some book where there was a world these people went to where magical society had been divided along gender lines. The women were associated with circles and I think the earth, and the men with straight lines and fire or some shit. I remember this because as a project we had to make games based on the book and I made some bullshit card game because I was good at drawing and used this as an excuse to draw people murdering the fuck out of one another with magic. (I relied heavily on my mom's Conan comics for help.) Anyway, I remember thinking the male magicians were self righteous pricks. (I also suspect this was not the main part of the story. Seriously if anyone remembers this please tell me what the crap I was reading.)
The Dark Tower: The Gunslinger by Stephen King. This is going to be a weird one to explain, I think. This was the original cut of the novel, which I picked up secondhand at a used bookstore in downtown Abbotsford. It's not a long book, and I started reading it on the bus ride home. It was early summer, so it was hot out. My bus stop was up a side road - I would get off and walk down a fairly quiet street for a few blocks before I hit the main drag, which I would cross and then continue into the less affluent residential area where our apartment was. I got off at my stop and kept reading as I walked to the main road because I just could NOT put the fucking book down. The first Dark Tower, especially before King re-edited it to make it fit the later books better, is a tale of utterly desolate magic. It's not hopeful.
I'm going to pause here to say that a LOT of Vertigo titles were inspirational to me as a teenager, even though at the time I used to get them via single secondhand issues. I hardly ever knew what was going on, but there was Constantine fighting demons, Timothy Hunter bitching about fairies, and Jesse Custer hunting down God the Almighty. This was all pretty anarchic shit to a girl now stuck in the Bible Belt.
The Diana Tregarde Novels by Mercedes Lackey. I had some online friends who tirelessly recommended fantasy novels to me, and although I tried several of them it just never clicked. The Tregarde novels, however, were set in the real world. I look back at the pseudo-history of Wicca included in the books now and I chuckle, but at the time Diana was sort of heroine I needed to read about.
American Gods by Neil Gaiman. A friend told me to read the book - "you're in it," she said. It remains one of my all-time favourite novels, and one of my all-time favourite depictions of Thoth and Anubis.
The Invisibles by Grant Morrison. This was one Vertigo title I'd missed when it was being published. I actually found the Barbelith forums before I read the books. I blame Pete for this one. Chaos magic used to be really sexy, you guys. ...even if it was still pretty goddamn phallocentric.
I think that everything we read becomes part of our mental compost heap, but these are certainly the titles that stick out as having had a conscious influence on me as a witch.