Farmer in the Sky

Warning: Wikipedia contains spoilers

In a future Earth where food is carefully rationed and Boy Scouts fly helicopters, a widower takes his son and new bride off-planet. After displaying heroics on the space ship, the boy is humiliatingly cut down to size by Scouts on his new home and struggles for acceptance. He volunteers to do the work of homesteading by himself, after his stepmother proves unable to live on the farm for health reasons.

First appearance of the remains of a lost (but advanced) civilization.

First appearance (in Heinlein's novels) of the concept of ecology bringing on a war.