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

The Number of the Beast, by the legendary author of the classic bestseller Starship Troopers, is one of the most audacious experiments ever done in science fiction.
Robert A. Heinlein's The Number of the Beast, originally published in 1980, follows the adventures of Zeb, Deety, Hilda, and Jake when they are ambushed by the alien "Black Hats" and barely escape with their lives on a specially configured vehicle (the Gay Deceiver) that can travel along various planes of existence, allowing them to visit parallel universes.
However, unknown to most fans, Heinlein had already written a "parallel" novel about the four characters and parallel universes in 1977. He effectively wrote two parallel novels about parallel universes. The novels share the same start, but as soon as the Gay Deceiver is used to transport them to a parallel universe, each book takes the readers to a totally different parallel world. From that point on, the plot lines diverge completely. Although some later passages may be found in both books, most of them are, fascinatingly, given a totally different context or perspective.
The companion parallel book was never published, and there have been many competing theories as to why (including significant copyright issues in 1977 due to some of the material used in the book). Over time the manuscript was largely forgotten, although it survived in fragments. A recent re-examination of these fragments made it clear that, put together in the right order, they constituted the complete novel that is finally being published as The Pursuit of the Pankera.
As both books are "parallel" books, they carry the subtitle A Parallel Novel About Parallel Universes.
So here it finally is, the other half of Heinlein's audacious experiment: a parallel novel about parallel universes.