It was huge before that too. Back in the mid-90s, my daughter was in the second grade. Suddenly she started balking at going to school. When I finally identified the problem, it turned out to be book reports being given at school by kids reading the scary R. L. Stine books. She was afraid—not of going to school but of leaving me alone at home in our upper-middle class neighborhood.
I always thought Goosebumps was more kids than YA, but the dividing line is always hard to find, and certainly Stine proved there was a fat load of cash in the under 18 market. In the early 80s when I was in the target market YA was Hardy Boys and Judy Bloom, and they kind of sucked. I’m glad the market is finally being addressed.