The book I'm most thankful for is Matilda by Roald Dahl.
In a lonely childhood, Matilda taught me that I could find friends in books. I felt like such an outcast and Matilda was the first person I met who was just like me. She loved to read more than anything and I did, too. She felt like the odd one out in her family and I did, too. She had a terrible time at school and I did, too. Suddenly, I didn't feel so alone anymore; I felt like there was someone out there who understood me, even if she only existed in the pages of a book.
Matilda also fired up my imagination and gave me hope that things would get better for me. I never developed telekinetic powers (although not from want of trying. I spent hours trying to move a carrot with my mind), but I learned that with time and patience, I might develop my own strength. When I saw Matilda meet and connect with her teacher, Miss Honey, I saw a model for a wonderful, supportive mentoring relationship. Throughout my childhood, I wished for a Miss Honey of my own and although it never quite happened for me, it lead me to become a teacher myself - so that I could be Miss Honey to someone else.
I've read many books since Matilda and related to many characters, but there's always something special about your first and I can't deny how much Matilda has helped shape the narrative of my life. For that, I'm thankful.