The Secret of the Ninth Planet by Donald A Wollheim is a YA science-fiction novel from 1959. Reading it was an exercise in nostalgia. It's a space adventure set in the solar system as it was conceived in the fifties. Most of it is now completely wrong--though it was accepted as being plausible theory when written. Mercury keeps one side to the sun all the time. Asimov wrote a science fiction mystery based on that assumption. We now know that it rotates slowly. Venus in the Fifties and before was frequently imagined as a hot, wet world not the violent poisoned maelstrom we now know it to be.
Mars, in the novel, of course has canals and even cities and inhabitants. This, in my opinion, is the most interesting section of the novel with its hive mind creatures. Even Neptune has an odd kind of life. But Pluto is described as a moonless planet the size of Earth which has been robbed from another star--far from the dwarf planet that we now realise it is.
Don't look for any psychological depth and be advised that
all the characters are male--
Wollheim was more important as a publisher and editor, He edited the first mass-market science-fiction anthology
The Pocket Book of Science Fiction {1943}. I have that book and a good few of the stories still hold up.