Long before it was called “Young Adult” literature, Robert Heinlein was writing fantastic sci-fi stories that would appeal to teenagers and adults.
I won’t go into his bio here (If you are interested you can Google him) but I will say he was an engineer and a very free thinker. His attitudes in regards to sex, marriage and society were as progressive as his take on technology, religion and politics.
His YA books like Have Spacesuit, Will Travel and Red Planet, Starman Jones, were light and exciting Sci-Fi tales that would thrill young readers while still holding the interest of seasoned adults. It was his more Adult offerings that really grabbed me. Stranger In A Strange Land is probably his most famous and most controversial. Door Into Summer, Starship Troopers, (much better than the movie) Glory Road, Puppet Masters (again better than the movies) I Will Fear No Evil, and Revolt in 2100, also push the boundaries of what is and was considered taboo subjects in the mid 20th Century America, when they were first published.
The underline theme of all of Heinlein’s novels was always, Peace And Love, will solve all of problems, either personally or globally. He had a glass half full view of the world. He was not blind to the problems we face as a nation and a planet, but he recognized that the same people that made the mess had the ability to fix it. We need his message today more than ever.