I've read most everything Heinlein wrote. Like all good sci-fi, his books are not about the sci or the fi. They're about human nature, sometimes tested in unnatural circumstances.
My Mom, who was not a big sci-fi fan liked "The Door into Summer". Nominally a time-travel book with several twists and a lot of Heinlein's humor, and also his love for cats.
"The Puppet Masters" is probably one of the best of the conspiratorial, trust-no-one stories, exciting, but lighter on the character development.
"The Star Beast" is one of Heinlein's juvenile books, although as with all his juveniles like "Have Space Suit Will Travel" or "Podkayne of Mars", the themes are adult, and the dangers are real. In "Podkayne", the main character dies heroically, which is unusual for a juvenile. "The Star Beast" is one of the funnier Heinlein juveniles involving a small alien pet. Think of it as somewhat analogous to the Star Trek episode "The Trouble with Tribbles".
I personally think one of Heinlein's best of the more hard-core sci-fi books is "Starship Troopers". He really explores a multitude of ideas here including why governments collapse and possibly how to prevent such a collapse, duty, honor, family bonds. He also posits some interesting sci-fi gear for his space marines including aero-pods allowing the marines to parachute onto a planet from orbit as well as muscle enhancing armor (think "Aliens" Power Loader Exo-skeleton).
Thanks so much. And we have a feral cat that we tamed, so there’s that. At this point I feel like I am going to delve into more than one of his books.