After reading Dick King-Smith's novel, Spider Sparrow, I am mystified as to how I should classify it.
I suppose it is a children's book. Amazon describes it as being for children ages 9 to 12. Dick King-Smith writes books for children.
However, as with any well-written book with a really good story (like The Book Thief, The Chronicles of Narnia, The Giver, etc.) this book is great reading for any age person.
The story takes place in an English farming community before and during World War II. Spider Sparrow was abandoned as an infant and left outside a shepherd's hut one night during lambing season. The shepherd, Tom Sparrow, and his wife had no children, though they very much wished for them. The Sparrows fail to find the mother of the abandoned infant, and end up adopting him. As the baby grows, it becomes obvious that he's mentally slower than other children. He is nicknamed "Spider" for the odd way in which he walks. Some of the locals make fun of him, some ignore him, and some treat him kindly, but he is loved unconditionally by his adoptive parents.
As he gets older, he exhibits a talent for mimicking animal sounds, although he cannot say much or understand well. The kindly owner of the farm on which the Sparrows work finds jobs for Spider to do. Spider's love for animals and their trust in him helps ease several difficult situations on the farm.
The ending is bittersweet, and I won't reveal it. But the entire story was so charming and sweet that I wish it was a more popular book.