(3) Base Camp
There are some hardcore political and scientific concepts being tossed around in The Moon is a Harsh Mistress: rational anarchism, catapulting materials through gravity wells, and the inner-workings of analog computers, to name a few. Yet, against all odds, the novel manages to present them all in a completely accessible manner.
Sure, there are a few doozy passages here and there, such as:
Luna's mass to three significant figures is seven point three six times ten to the nineteenth power tonnes. Thus, holding other variables constant including Lunar and Terran populations, the present differential rate of export in tonnes could continue for seven point three six times ten to the twelfth years before using up one percent of Luna—round it as seven thousand billion years. (6.173)
If you read that and your mind goes cross-eyed, don't worry; you're not alone. Thankfully, there will often be a character in the novel with the same stupefied response as you, requiring another character to explain the technical in more user-friendly terms.
In this way, Heinlein's novel and its readers get to have their technical cake and eat it, too. Most anyone can enjoy the story and grasp how all its tech mumbo-jumbo fits into the plot, but those who enjoy technical subjects such as science and engineering receive an extra layer of awesome to gobble down.