The Mushroom Planet Series

This is a group of children’s books by Eleanor Cameron. Two boys build a space ship and fly to a planetoid named Basidium. There, they meet the Mushroom People and have adventures.
Books in the The Mushroom Planet series:
Stowaway to the Mushroom Planet (1956)

David and Chuck take Theo Bass, Tyco Bass’s cousin, to the Mushroom Planet, unaware that a young astronomy professor has stowed away on board.
Mr. Bass’s Planetoid (1958)

David and Chuck are off in their spaceship to rescue the world from the Brumblitron.
Jewels from the Moon and The Meteor That Couldn’t Stay (1964)

Two short stories in which David and Chuck take a dream journey then help Mr. Bass recover a brumblium meteorite. This was written as an elementary school reader.
Time and Mr. Bass: A Mushroom Planet Book (1967)

Chuck and David are drawn into a battle against evil as Mr. Bass searches for the stolen necklace of Ta that can protect the Mycetians from destruction.
A Mystery for Mr. Bass: A Mushroom Planet Book (1968)

David and Chuck discover Mycetian fossils along the sea shore, and make an emergency trip to the Mushroom Planet to save Prewytt Brumblydge.