worlds of the imperium combines alternative realities and alternative histories in a classic 1960's science fiction book.
keith laumer introduces a novel idea of alternative reality that could be oh so true. unlike other time travel and alternative realities that always seem to be unlikely, the authors idea was unique.
if parallel universes are are not linked in any way to your universe then is this books idea a sort of parallel reality?
you can see where the idea for the recently published - but the idea for it was back in the 1980's - long earth series (review) of sci fi books comes from.
and that is one of the issues with the book. the idea was unique and the set up is great but laumer does not quite fulfil what could be done with it. the book is very short though, so any more plot and that would have extended it.
after reading the long earth book you see what could and can be done with the sideways alternative realities. but as this is the first book in the imperium series the author might show what can be done in the other books.
the worlds of the imperium book is another vintage keith laumer book - full of humour, improbable plot coincidences that you dont mind as its a ripping yarn, and great characters. this book also has a bit of good hard science fiction ideas and technology.