Basically, after Heinlein's wife passed away, and they were going through his stuff, there were notes for a book that R.A.H. never wrote. He had 8 pages of detailed notes (one of which was lost before Spider Robinson got them). As far as I know, noone knows why he never wrote the book. But Spider Robinson was asked to write the book, using the original notes and that is what he did.
I'm not going to talk about plot - so no spoilers story wise. Though if you are really, really looking forward to reading this book, you are insane to be reading anything about it. But whatever. And let me just say, I'm not to good at writing critical reviews of stuff. That will be self-apparent here in a minute, but I just wanted to get out there, that I know it. I'm doing this for fun.
A little other self-disclosure would probably be helpful. I've read just about everything published by R.A.H. (I haven't read 'For Us the Living' and my have missed any short stories if they were never published outside of a magazine.) I am a big fan of the juvenile stuff. The later stuff isn't nearly as appealing to me. I think that part of this is not just what he wrote but how he wrote it, due to his perspective at the time. I love references to slide-rules and other stuff that reminds me of when they were written. I enjoy the lack of profanity and a certain level of naivete that is there. And I enjoy reading his political/philosophical musings that he slips in, but I don't really buy into a lot of it.
So I guess first off, is this a Heinlein book. For me - yes and no. It probably very much written the way he would have written if he were alive today and didn't have to cater to 50's sensibilities. So you don't find the lack of profanity, drug use, sex, etc. Though the sex side is pretty light.
But a lot of the vocabulary is Heinlein. There is also something that a lot of his protagonists share. It is a guy, who is extremely intelligent, but stupid in regards to himself and relationships. If the question is about any number of subjects, he is smarter than many, but if it involves himself, he is clueless. Things that the reader figures out immediately, have to be spelled out in painful detail by another character.
A lot of what Heinlein did was still pretty novel at the time. Many of those ideas have been used a great deal since, and are much better known. I'm not saying that he originated them, but used them before they had been used a lot. For example, when I read Starship Troopers for the first time, I was not familiar with the idea of powered armor. Now it has been used quite a bit and I don't think would be such a novel idea to many people. So the plot of this story might have been much more novel when it was conceived, but not so much now. (This is not a problem just a difference.) If you want to read another "Have Spacesuit Will Travel" or "Starman Jones", this really isn't it. But it is a good book that is fun to read. Like those others, but with some modern terminology and much less sanitized.