Today was the 100th anniversary of the birth of one of the world's greatest science fiction writers, Robert Heinlein. Talk to almost any current sci fi writer, or any of the people involved with the efforts to exploit space, and most will tell you they grew up reading his stuff. I sure did. His wrote excellent juvenile sci fi. Books like Have Space Suit, Will Travel ; Space Cadet ; Rocket Ship Galileo ; and Tunnel in the Sky will sometimes seem formulaic when read today. What you have to remember however is that Heinlein was usually the one who invented the formula. The science can seem quaint and outdated, but again when you realize these works were written in the late 1940's and the 1950's, it helps to put things in perspective. These early works did more than open up sci fi to a wide audience of young boys (and eventually girls), it helped sci fi make the transition from pulp magazine to where it is today.
Perhaps his most famous book is Starship Troopers . Ignore the movie, it is NOT a faithful rendition of the novel. The book is not about space combat. It is often called fascist, but that is a misunderstanding of the book. It is about the responsibility of being a citizen in a republic, and the dangers of mass democracy. The book doesn't glorify the military, it glorifies responsibility.
The transition from his juvenile novels to his adult novels can be quite jarring. (Especially you make that transition while still a juvenile as I did.) His adult novels deal with adult themes, including sex and drugs. (some of his sexual themes include not only incest and homosexuality, but sex with clones of one self) Many consider him one of the fathers of the sexual revolution and counterculture movement of the 1960's. Some of these books are Stranger in a Strange Land ; The Cat Who Walks Through Walls ; The Moon is a Harsh Mistress ; and Time Enough for Love. These books revolutionised science fiction. No longer was sci fi relegated to ray guns, bug-eyed monsters and spaceships. These books grapple with all of the problems of humanity in a thoughtful and logical way. Whatever else, Heinlein will leave you thinking.
These later works do something very rewarding for the Heinlein fan. By various plot devices including time travel, cloning, immortality and the creation of different dimensions, Heinlein ties almost all of his works into one coherent universe. It was a welcome surprise to be reading one of his later, adult, titles and have some of your favorite childhood characters re-emerge. The practice of creating a universe of your own to explore and write in is fairly commonplace today (Asimov's Foundation universe, Weber's Harrington universe, and Niven's universe as examples), but it wasn't when Heinlein did it. Most books were either stand alones, or parts of an explicit series. Heinlein was further challenged in that he created his universe retroactively, long after his most of his books had been written.
Lastly Heinlein has had an effect on the very vocabulary we use today. Many people use his acronym TANSTAAFL (there ain't no such thing as a free lunch) with no idea of where it comes from. It is included in numerous economics and political science textbooks. A less commonly used word, but still seem from time to time is grok. Also, like many sci fi authors he has had his work adopted by scientists and engineers, like the term waldo.