Helen Cresswell is much underrated - a huge contributor to the cultural life of children growing up in the 60s, 70s and 80s, both with her own superb books and her adaptations of other peoples books for TV. I loved the Bagthorpe saga especially, because it was so funny, but she also did some seriously chilling supernatural stories, eg Winter of the Birds. A genuinely great writer who deserved more fame than I think she ever got.
I never read the Swallows & Amazons books, despite my dad going on about how much he had loved them as a boy. He more successfully introduced me to Richmal Crompton’s William books, which I read over and over and over... They still make me laugh as a grown-up.