Podkayne of Mars is a very sexist and unsuitable story for a modern girl. I wanted to be an astronaut when I was young, and my teacher told me that girls could only be nurses, teachers, mommies or -- if they were really stupid -- secretaries. Heinlein echoes this primitive and demeaning opinion when Podkayne debates becoming a starship captain "that nobody will hire" or confining herself to managing the nursery in a starship, a suitable job for a girl. Hard to believe that in the far future we will still run into this kind of piggery cramming us back into our biological role and that alone!
My first Heinlein books were "Tunnel in the Sky" (survival school on another world) and "Time for the Stars" (an experiment in which twin telepaths are used as communication equipment aboard a spacecraft.) The former was quite good for its time and showed girls equally good at managing daily life without amenities.
I stumbled across "Space Suit" in the threadbare library of my junior high school in 1970. I was 11. Been an RAH fan ever since.
Patricia Wyant Reisfeld (sp?) lived in my reverie when I was still terrified of girls. (You'll have to read the book to know.)
About half of what Heinlein wrote is laid against a common background, sometimes known collectively as his "future history."
Most of that, from "Lifeline" to "Methuselah's Children" is complete in one volume, if you can get your hands on "The Past Through Tomorrow."
In the late 70's, he wrote a capstone to all that, a hefty tome called "Time Enough For Love." It's all fine reading, I think his best work, but some of it is at least PG-13, even today.
Heinlein's deepest penetration into lesbian sex (make up your own joke here) was with "I Will Fear No Evil," about an old man whose brain is transplanted into a young woman's body. RAH actually danced around a lot of societal taboos with this approach. And the political commentary in "Evil" is unsurpassed.
All that said, I could not get 100 pages into "Number of the Beast" without my eyes glazing over. Wish I had stopped with "Friday." You might wish you had stopped reading this several paragraphs ago.
Have a good weekend.