Published by Heinlein in 1980 . The book is written as a series of Diary entries by the 4 main characters. The main characters are traveling ( perhaps fleeing is a better term) between universes after multiple murder attempts on them fail in the 1st few chapters. They observe differences between the universes , some minor just as one when the letter J does not exist and others more extreme ( i.e. the British empire and the Russian empire battling it out on Mars in steam powered ornithopters ). The eventually find a connection between novels and universes and end up in the land of oz and e.e.doc smiths lensmen universe. They eventually meet up with Lazarus Long ( yep he shows up again) and then subsequently nearly every character ever introduced in Heinlein’s novels ( at least it seems so).
The number of the beast ( 666 from the bible) is reinterpreted as 6 to the power of 6 to the power of 6. a really huge number that equates to the number of possible universes on any given axis of the multidimensional geometry that was thought up for this story).
Personally although I enjoyed the book i felt that it lost focus towards the end ( after they had joined up with Lazarus) and just dissolves into a conference of speculative fiction authors at the end.