Where the Sidewalk Ends is one of the first books I remember opening. I did that a lot. Looking back, I think I was lead to it by my dad, which stands out because I was mostly allowed to shape my own tastes as a kid. When they were young, my Aunt was diagnosed with Leukemia. It's a subject that I've only heard him talk about a hand full of times. The fact that she wrote to Silverstein and got a response is one of the few things that has bubbled to the surface and been put into words.
What a treasure that letter must be. Like you, I was introduced to his works at a young age, beginning with Where the Sidewalk Ends. I have children of my own now and I read his work to them. I've even begun writing my own children's poems. #childrenspoemsbytng I would credit my love of lyric to him. He was amazing.