Most readers did not realize in 1980 (when it was originally published) that the novel had a sister book, written in 1977, that was never published. That book is finally being published under the title The Pursuit of the Pankera. .
Both novels deal with parallel universes, share the same main characters and have the same first one-third of the book. However, from that point on (after they make a jump to a parallel universe) the novels diverge completely. .
And here is where the second part of the experiment comes in. While The Pursuit of the Pankera continues the adventure in a very customary Heinlein manner, reminiscent of his earlier works, The Number of the Beast becomes something very different. .
On surface, the book is about two men and two women who are attacked by aliens and then embark on roller coaster ride of an adventure through a myriad of universes. But as Jack Kirwan wrote in The National Review, "describing The Number of the Beast thus is like saying Moby Dick is about a one-legged guy trying to catch a fish."
The Number of the Beast is a homage to science fiction, to his friends and to characters used in other books, also serving as a parody and a lesson to anyone willing to listen, in a way only Robert A. Heinlein could have presented it.