It wasn't till my dad wanted some father/ daughter bonding time and decided to read aloud Tolkein to me that the world of books was really opened to me. Not only did we read together but he expounded his countless theories about how these books related to gospel themes which sparked a myriad of discussions that lasted late into the night. From there dad was able to direct me to a whole world of books that he had grown up loving and reading. I never finished The Hitchhiker's Guide but I really enjoyed the first couple books of Another Fine Myth. Then the Harry Potter craze started and after the third book came out, I yielded to a close friend's insistence to give them a try. The books gave not only my dad and I, but by this time several of my siblings, something to read and talk about together.
At this point I began to read, almost bookclub style, with my two sisters closest in age. We read I believe all of Tamora Peirce's quartets, and things like Johnathan Shroud's Bartimaeus trilogy, Shannon Hale's fairy-tale like YA fiction, and later Chistopher Paolini's Inheritance Cycle. We would read and swap and discuss and I believe the books not only shaped our imaginations but also, in part, who we wanted to become. I was very strongly influenced at a young age by Peirce's strong feminist heroines and Shroud's sarcastic characterizations.
As I entered high school and then a running start program at the local community college, I began to realize an interest in texts beyond YA science fiction and fantasy. I never realized till I later transferred to BYU exactly how many literature classes I took as electives (and thankfully they all transfered). I look back and realize a lot of my conversation was about the short fiction, poetry, and drama I was reading for these classes, but at the time I didn't even think of myself as a big reader.
Probably everything I read had some effect on me but I don't know that I can pin down one exact text that I felt significantly changed me. I read texts differently now than I did at 11 or 17, but when I do reflect back on those stories, I realize how tied my youth was to them. My reading has definitely slowed significantly over the years. Now most of what I read is for school. I have great long lists on goodreads and in my head of books I want to read after school is finished though.
One book that my mom reminded me did have a big influence on me was an LDS authored book entitled Where Do I Go from Here: Finding Your Personal Mission as a Young Adult Woman. I read it after I graduated high school and using it as a self-help type book, I was able to recognize my individual talents and personality and develop confidence to move forward with life as a young adult. That's that best candidate I can think of for a book that affected me seriously enough that I made changes in my life. I recommend it to all the young LDS women I know who are recently graduated.