I scanned the forum looking for a post discussing my favorite author, and not finding it, I decided that it must then fall to me to bring up this great man.
Heinlein is thought of as a ground breaking science fiction author, who was the first to cross the barriers of genre and produce a modern classic novel from the Sci-fi genre. I speak, of course, of Stranger in a Strange Land. While the story of Valentine Michael Smith, as a fish out of water and a human raised by aliens and then returned, Stranger is a story anyone who has felt like an outsider at some point in their life can relate to.
What's not often discussed about Heinlein is that he was one of the first authors to bring the subject of sex change into the serious fiction arena, rather then the smut peddlers rack. In fact, the three Gender Bending charachters of Heinlein's universe lead one to believe he was well acquainted with someone who had changed genders, simply from the delicacy with which he writes about the subject.
The earliest example was All You Zombies.... which, I admit, is a bit of a ridiculous plot, and doesn't make sense from todays medical knowledge, but it was written in the 1930's. The main charachter in the novel loops back through his own life after having a sex change, and fathers himself through his earlier female self... Then kidnaps himself to be raised in the future. The possibility that he was fertile as both a male and female is attributed to having an XXY Karotype (Commonly refered to as Kleinfelters.) While the medical facts don't check, it was a short fiction intended for a pulp magazine audience, but it was groundbreaking in that it dealt with a transgender individual in a non-pornographic setting.
As Heinlein's audience grew up, so did his writing style. He stopped being subjected to strict editing by his publishing house to tailor his books to the teenage boy category, and was allowed to write full length novels dealing with adult subjects.
One of my favorite books, and the one I'm currently re-reading, is I Will Fear No Evil, the story of a rich old man who determines that if he can't take it with him, he's not going to go. Johann Sebastian Bach Smith is almost a century old, and he'd made the mistake of allowing the medical establishment hook him up to the machines that make it damn near impossible for him to die. Since the machines will keep him trapped in a miserable, painful existence for many years to come, he cooks up an elaborate scheme involving an Australian doctor who had succesfully transplanted the brains of 2 monkeys. He sets his lawyer to sign up people with his rare blood type ahead of time, so that if they are in an accident that kills the brain without killing the body, they will effectively donate the body to Johann. Well, a donor is found, and amazingly (well, the novel would have been very short otherwise) the operation is a success. Johann wakes up in the body of a 29 year old female. I won't ruin the rest of it with spoilers, if you haven't read this book, I highly suggest it to this audience. It's a fun adventure, and a dream I'm sure many of us wish could come true.
The final example of a Trans charachter in Heinlein's work is actually mostly a footnote. His early work featured prominently Andrew "Slipstick" Libby as the partner in crime of his main protagonist, Lazarus Long. Much later, he brought Andrew back, as Elizabeth - Lazarus jumped through time and recovered the corpse of Andrew to be rejuvenated using superior modern technology (Circa 4000 CE) and during the rejuv process, it was discovered Andrew was actually XXY (A bit of a recurring theme, but he went 50 years between usages, so I think we can forgive him) His doctors give him the option, and he chooses most emphatically to be Female - and Elizabeth is transformed from a mousey bookworm of a man into a vibrant, happy, and emphatic Female. The charachter recurs in The Number of the Beast and The Cat Who Walks Through Walls. It's the portrayal of Elizabeth that truly makes me feel that Robert must have at least known a Trans women, and been not only sympathetic, but very empathic with her issues.
Heinlein has many other works that are worthy of reading, and discussion, but these are the primary ones that are very relevant to this forum and are good starting points for someone who is interested in reading up on this master of the craft.