"The Pursuit of the Pankera: A Parallel Novel About Parallel Universes" by Robert A. Heinlein and an introduction by David Weber

"The Pursuit of the Pankera: A Parallel Novel About Parallel Universes" by Robert A. Heinlein and an introduction by David Weber
 https://www.amazon.com/Pursuit-Pankera-Parallel-Novel-Universes/dp/1647100291/
Book number four in a very loose series of six science fiction books.  I read the well printed and well bound trade paperback published by CAEZIK SF & Fantasy in 2021 that I bought new in 2021.  The book was found in Robert Heinlein's stored items in 2017 in many pieces and several editors put it together over three years for publishing.  I plan to reread "The Cat Who Walks Through Walls" and "To Sail Beyond The Sunset" soon.  I have yet to decide if I am going to reread "The Number Of The Beast".
This book and "The Number of The Beast" are exactly alike through about first third of the books.  After that, they are radically different. The current theory is that this book was written first and Virginia Heinlein vetoed it.  Plus there were copyright problems.  So RAH threw away the middle and end and rewrote it.
I need to reread Doc Smith's Lensmen books.  I did not remember Worsel the Venusian dragon Lensman nor many of the other Lensmen.  But I read those books over fifty years ago and lost several of them when my house was flooded in 1989.  Anyway, I really enjoyed the first third of the book and the last third of the book.  The middle third dragged just a little bit, mostly because I have forgotten most of the Edgar Rice Burroughs's Mars books and I never read the Oz books.
If you are offended by sex, and I mean lots of sex, in a book then I would advise you to stay away from this book and series.  Except for the first book in the series, "Methusalah's Children".  All of the books, except the first book, have group marriages in or mentioned in them which was first expounded by Heinlein in "The Moon Is A Harsh Mistress" book.
The book series is
1. Methuselah's Children
2. Time Enough For Love
3. The Number Of The Beast
4. The Pursuit Of The Pankera
5. The Cat Who Walks Through Walls
6. To Sail Beyond The Sunset
There is a much better good review at "Long-Lost Treasure: The Pursuit of the Pankera vs. The Number of the Beast by Robert A. Heinlein" by Alan Brown
 https://reactormag.com/long-lost-treasure-the-pursuit-of-the-pankera-vs-the-number-of-the-beast-by-robert-a-heinlein/
Jo Walton says that "The Number Of The Beast" is not Heinlein's worst novel by far, but, it is too long.  She does not have a review for the Pankera book that I have found.
    https://reactormag.com/heinleins-worst-novel/
My rating:  5 out of 5 stars
Amazon rating:  4.4 of out 5 stars (2,648 reviews)
Lynn