In James Nicoll's review of Niven and Pournelle's Mote in God's Eye, he posted a link to a Heinlein
sampler of the Virginia Editions, which contains the revision letter Heinlein write to Niven and Pournelle. It runs 16 pages in printed form, and is fascinating. I can see Heinlein's fingerprints all over the final version, especially in the excising of a long prequel. (I read said prequel in a separate publication, and was totally underwhelmed. And even if it had been of more general interest, it didn't belong in the main book.)
The sampler also contains some fiction, including "Beyond Doubt," his only fiction collaboration (with Elma Wentz). He wrote a three-page outline, she wrote the story, and he revised and shortened it. It's justifiably obscure.
More uncomfortable are the political pieces.
There's an unsent letter to F.M. Busby on the subject of race that predates the Civil Rights movement, and comes down to 24 pages of "Fuck you, Jack, I've got mine" -- a phrase, ironically, that I first encountered in bowdlerized form in the pages of Analog, though I don't remember the story. The letter is so very, very wrong, and I couldn't possibly attempt to rebut it; I'm glad that the future has pretty much done that for me.
There are also a couple of essays on the necessity for a world government in the atomic age. He sets out his thesis clearly: "The Next War will start with the devastation of American cities by surprise attack with atomic weapons launched from thousands of miles away at super-sonic speeds; against such attack we have no effective defense and no real hope of developing a defense." His claim was that the choice was between going it alone, which meant dispersing the American people so that such an attack would limit the harm, or creating a world government to manage the atomic superweapons. While he probably had a certain amount of confirmation bias, it was a reasonable conclusion from the data available. (I was a child of the 1960s, and pretty much took it as an article of faith that everything was going to go to hell at some nebulous point in the future. I also took it as an article of faith that Social Security would not be around by the time I was of age for it, and it's starting to look as if I'll be wrong about that one, too.)
The entire Virginia Edition series is something I can't afford (and don't have room for), but this is the "warts and all" type of content I had been looking for -- and was very disappointed by the lack of -- when Grumbles from the Grave was published. Kudos to the Heinlein estate for making this sampler available for free.