I’ve read his autobiography, and his nonfiction books on Russia, as well a couple of biographies about him. He was an interesting and remarkable man.
You could say that the Swallows and Amazons series is more about children, than for children. Even though they were huge bestsellers as children’s books, they have always been enjoyed even more by adults than by children.
They’re also highly recommended for people who like ‘British shit’. And sailing! 
In many ways, Arthur Ransome was far ahead of his time. One of the main characters, Nancy Blackett, definitely has gender dysphoria.
The most remarkable thing about it is that nobody considers it remarkable. Everyone, both children and adults, accept her as she is without comment or judgment. Nothing is ever mentioned or discussed. There is never any moralising.
You simply notice as you read (if you’re observant) that she likes to dress in boy’s clothes, that she avoids activities like cooking and sewing that she regards as ‘women’s work’, and that she identifies as male in all significant ways. It’s presented as inherent from birth. ‘Nancy never liked playing with dolls when she was small’, her mother says.
She’s better than any of the boys at sailing, camping, and any kind of outdoor and practical activity. More than that, she’s a natural leader, automatically in charge in any group of children, bossing adults around as well as children, self-confident, comfortable in herself, and upbeat.
In general there are more girls than boys in the books (there are about 14-15 children that you get to know well over the series!), and girls are mostly stronger characters than boys.
Another character, Dick Callum, who appears in six of the twelve books, definitely has Aspergers or high functioning autism spectrum disorder. These books were written at a time when ASD was utterly unknown, yet Arthur Ransome describes it with great accuracy, understanding, and empathy. Dick is also accepted without comment, just as he is, by both adults and children.
The character was no doubt based on a real child that Ransome knew, as most of his characters were. Ransome wrote what he saw, and he had the ability that all great children’s authors have of being able to see the world through the eyes of a child.
I’ve always thought that he’s highly underrated as a writer.