The Big Time
by Fritz Leiber
is free on iTunes. Hugo award winner in 1958.
https://itunes.apple.com/us/book/the...ign-mpt=uo%3D4
Below is the blurb:
Have you ever worried about your memory, because it doesn't seem to recall exactly the same past from one day to the next? Have you ever thought that the whole universe might be a crazy, mixed-up dream? If you have, then you've had hints of the Change War.
It's been going on for a billion years and it will last another billion or so. Up and down the timeline, the two sides--"Spiders" and "Snakes"--battle endlessly to change the future and the past. Our lives, our memories, are their battleground. And in the midst of the war is the Place, outside space and time, where Greta Forzane and the other Entertainers provide solace and r-&-r for tired time warriors.
Edit:
I found it on manybooks.net also where there are lots of formats to choose from:
http://manybooks.net/titles/leiberf3225632256-8.html