This battered old copy of Charles Kingsley's The Water Babies was illustrated by the twin sister team of Anne and Janet Johnstone. There are actually eight colour plates in the book but, as ever, it's the black and white work with resonates with me more. I'm fairly sure there's a strange bibliographical quirk to be sorted out here. When I was Googling around this book I saw illustrations which were clearly not in my copy (The Heirloom Library, no date) and Wikipedia tells me that Anne produced illustrations for The Water Babies in the 1980s, on her own after the death of her sister, and yet this copy is clearly credited to both of them. I'm pretty sure it must have been a re-release with a few extra illustrations and that the original set were created by the both of them before Janet's death.
The sisters were best known for their illustrations of Dodie Smith's 101 Dalmatians and I think it's possible to see something of their influence even in the Disney-fied cartoon. Their style was so very 'of the period', the elfin faces and big eyes, one has to wonder if there is any connection between this 1960s/70s style of illustration (not just from the Johnstones) and Japanese manga style characters.