There are so many books that fall into this category for me, not the least of which was Texas Wildflower, my first published novel. But in my Book Brahmin interview, the first answer that came to mind is this one:
Book that changed your life: Yertle the Turtle by Dr. Seuss. It was the first book I bought with my own money, and it contained one of my earliest writing lessons—go big or go home. At age 7, I had no idea the subtext was meant to be a condemnation of fascism.