story identification - Boy who befriends an alien whose spaceship is disguised: a book about friendship

I am looking for a novel for my children that I used to read in the early 70s at the library. It's about a boy who befriends an alien whose spaceship is disguised as ordinary things. Part of the story is that when the alien returns (usually in a bigger and better ship) his spaceship is disguised differently. I think it's a series. It was a great adventure and I couldn't wait to go back to the library. It's definitely not Heinlein; it had illustrations and was found in the Children's Section. I seem to remember he helps the alien (who is also young) in the beginning and by the time the adventures get going he is visiting his planet and meeting other aliens. It was a great boyhood / alien / adventure / school age friendship children's novel. It was a bright story, nothing dark. The perfect novel for my children's age of wonder.

Answer

The Space Ship Under the Apple Tree.

Edit: A few story elements that I remember from when I read the books 30+ years ago:

• In the first book, the spaceship is a UFO-shaped ship that is hidden under an apple tree but otherwise obviously a spaceship.


• In the second book, the spaceship is disguised as a car, but with an invisible shield around it. The family's chickens enjoyed sitting on the invisible shield, the protagonist speculates that it lets them feel like they're flying.


• The protagonist (Marty? Or was that the name he gave to the "Martian"? I don't remember) suggests going to get something at the general store. The alien refuses, not wanting to interact with General Store of the human military.


• In one of the later books, the boys return to the alien's home planet. In the dense spacelanes near that planet, the ships all carry a strong electrical charge, so that they repel each other and are thus unable to get into accidents.


• The smaller ship that comes down to Earth docks with a larger ship in orbit to take the boys back to the homeworld.