The Number of the Beast: A Parallel Novel about Parallel Universes

The Number of the Beast is a mind-bending experiment by one of the greatest writers in science fiction who ever lived and the author of the classic bestseller, Starship Troopers. It is a parallel book about parallel universes.

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 Beastbecomes 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.