If we're allowed to name a series, I would say the Dr Doolittle books. I loved the idea of a pottering, slightly stern old man who studies animal languages and becomes an animal doctor. The way that he and Tommy, his apprentice, are scoffed at by their fellow townsfolk, yet revered by animals all around the world was wonderful to me as a child. In fact, about 10 years back, I went on A ebooks and collected the complete set of 1940s/50s hardback editions and re-read them. I can see why the books aren't more popular, and didn't become staple reading for Millennials, because the casual racism that was commonplace at the time of writing doesn't sit well any more. Prince Bumpo is an out and out black stereotype, both in the way he acted and was drawn.
It's interesting how the final story, written just after WW2 and finished off by a relative after Lofting died, has a much darker, more introspective tone than the others. The casual racism has gone. The doctor is no longer described as eating sausages and bacon, in front of his pig pal. I can't help wondering how amazing the stories would have been, had Lofting started writing them 30 years later.