I loved the Mushroom Planet books! Totally forgot about them.
I think the first MP book was published around 1954, and I believe these books did a lot to inspire young people to become interested in space and space travel. I must have been around 7 or 8 years old, when I found them in the school library.
Another fan has created a neat website:
http://peyre.x10.mx/MushroomPlanet/index.htm#Flight