The Number of the Beast is a
novel by Robert A.
Heinlein.
It is also a Song by
Iron Maiden and other things, see
number of the beast and
666.
The book by Heinlein is an interesting experiment on
multiple solipsism, the idea that reality is just a shared illusion and characters have the same reality as their creators. It features characters from
Methusala's Children,
Stranger In A Strange Land, The
Wizard of Oz and many other
science fiction and
fantasy books.