By Robert A. Heinlein and Spider Robinson.
It's hard to judge a book like this--a plot written by the late Heinlein (it's definitely a Heinlein plot...oh, it's a Heinlein plot), but the story was actually written by Spider Robinson, a man who much admired Heinlein, but who has entirely different sensibilities. He wasn't instructed to write a Heinlein book, either--just to write the best Spider Robinson novel he could, using the Heinlein plot as a skeleton.
It's like watching a medium really channel a ghost. Fake mediums convince us with the absolute unquestionability of the verisimilitude of the spirit whose messages they carry back to the world of the living. A real medium would act, I imagine, more like Whoopi Goldberg in Ghost, speaking in his or her own voice, telling the ghost to shutupshutupshutup all right already, what he wants you to know is that... Kind of spooky in places, actually, when you can tell that Spider would rather be doing anything with the plot but what Heinlein's spirit is forcing him to do...
I enjoyed it, but it's hard to give unequivocal praise to a book that doesn't wholly live its own life, but lurches around possessed at times. If you don't like Heinlein or Spider, don't read this book; if you don't like both Heinlein and Spider, don't read this book. If you aren't prepared for a few ectoplasmic floops here and there, don't read this book. If you're in a mood to drink up the essence of a dead guy, celebrated by someone who doesn't always agree with him, please do. I kept thinking of Spider, boiling up the ashes of Heinlein, knocking him back: "Needs salt."