Also see "The Number Of The Beast"
That was one of his weird ones..............had characters from his other books in it...............
"The Number of the Beast" started out great, and then Robert Heinlein lost his freaking mind before he finished the book.
He seriously looses his grasp of reality. I think he was suffering from his physical ailments at this point and simply couldn't keep his thinking rational. He may have had a stroke or something between the start and finish of that book.