In 1977, when this book was published, I was 10 years old - the same age as Jess and Leslie in the book. And if I had read the book that year, I think I would have found a lot in common with Jess - the inability to fit in, the interest in things artistic and fantastic, but also the need to keep that side hidden for fear of being made fun of. Yes, I think 10 year old me would have really enjoyed this book.
Reading it at almost-50, I do have some issues - the first part of the book moves so slowly, and it's almost formulaic in the "local boy has no friends but a new kid moves into the old house next door and teaches him all about life". And I knew (I just knew!) that something bad was going to happen to Jess or to Leslie. But the events that do happen are so beautifully and tenderly written. And the last couple of paragraphs had me literally in tears.