So many... In different ways, but the one that changed me the most profoundly was "The mists of Avalon" by Marion Zimmer Bradley.
I read it as a depressed 15yr old, in a time when I read everything I could get my hands on. From cover to cover. Often doing nothing else but going to the bathroom, eating and sleeping from exhaustion once I started a book until it was finished.
This book was so bad that I gave myself rules so that I wouldn't waste another 900 pages of reading on something so utterly uninteresting.
It made me suspicious of text and stories in a way that nothing else has.
I'm still grateful for having read it, it did teach me something.
