Variable Star is by Spider Robinson, based on a manuscript Heinlein never finished.
Robinson has promised to donate half the proceeds to the Heinlein Prize.
Does it get any better than this? 
Well, I still hope one day they find Asimov’s manuscript for “The Cosmic Corkscrew”, so yes, I guess it could get better. At least for me.
Being a Heinlein fan too, I understand your enthusiasm. However, I gotta say I’m not optimistic. I can’t think of a single posthumous collaboration that I ever really liked. In particular, I think of August Derleth’s posthumous collaboration with H.P. Lovecraft, and I groan.
On the other hand, if they had to do this, they couldn’t have picked a better man than Spider Robinson for the job.
I’m looking forward to it with guarded optimism. I was highly dissapointed with the last Robinson book I read, Callahan’s Con (think that was it). It felt like it was just thrown together following a formula. He’s taken the talking animal gimmick way further than it needed to go, and he killed off a main character for reasons that are not entirely clear. Maybe doing the Heinlein book will recharge his batteries. When Spider is good, he’s very good. When he’s bad…well, he’s still tolerable.
I think he might have done something different with Erin Stonebender. She’s not only an all-knowing supergenius from the day of her birth, but now she can teleport and time-travel with no equipment. We’ve discussed that kind of thing in this forum before – if your hero/heroine is too powerful ever to run a serious risk of losing, where’s the suspense?
As it is Spider doing the job, I have high hopes it will be a good and entertaining book. I did not realize it was due out in September, thank you for the OP.
Jim
I have mixed feelings about this one. Oh, I’ll buy it and read it the second it hits the stores, but…Oakminster and BrainGlutton have hit my major concerns. On the upside, maybe RAH left detailed notes for the conclusion and Spider can’t mess with it too badly.
Prepare for the worst – what if the finished product combines the worst aspects of Heinlein with the worst aspects of Robinson? Having something like that cross my field of vision would be painful.
I’m both enthusiastic and terrified. Heinlein was an ardent individualist libertarian and Robinson is a collectivist libertarian. I love them both, but I fear Spider may “hippyize” things too much.
That doesn’t mean I won’t be buying it, of course.
I haven’t read a great deal of SF since I left the Navy in 1991, but I particularly remember a Robinson collection in which he took time and space to seriously unload on people who dared to criticize the old guy. Frankly, I found his degree of Heinlein-worship to be somewhat off-putting.
So, Baldwin’s concern strikes me as being valid, from my fifteen-years-out-of-date perspective. OTOH, maybe in the past fifteen years Robinson has come to realize that Heinlein has a “worst elements of” to be avoided.
There’s an old story that Marilyn Monroe once offered to have Albert Einstein’s baby.
MONROE: Think of it, professor! The child might have my beauty and your brains!
EINSTEIN: But, my dear, what if it were the other way around?!
Here it is. FWIW.
Curiously, however, I can’t seem to get into Spider Robinson’s official website, which until recent was freely accessible to all with an Internet connection. It now requires a username, password, and domain name, and no clues are offered as to how to acquire or even buy any of those.
I have a mailing address for Robinson, so there! 
Share?
Please? 
I’d also say that I’m cautiously optimistic. In the link to Amazon.com in the OP one of the reviews states:
I love John Varley’s writing. And I think his review is probably reliable, given that he obviously likes Heinlein (understatement - ref. Steel Beach and The Golden Globe and the storyline with the Heinleiners).
And wouldn’t a Varley/Heinlein combination have been interesting?
How detailed/long was Heinlein’s outline, anybody?
Sir Rhosis
Dunno. The Amazon page doesn’t say and Spider’s website is for some reason inaccessible. This (rather negative) review just calls it a “detailed outline.”