Coming of the Quantum Cats — Frederik Pohl

A brilliant novel of alternate universes by an award-winning science fiction master
 
A breakthrough in quantum physics has shattered the boundaries between alternate worlds. History is in chaos as billions of possible futures collide. As a conquering army mounts an invasion of neighboring realities, a handful of men and women from a dozen different timelines risk their lives to safeguard an infinity of worlds.

Blending thrilling suspense with brilliant scientific speculation, Frederik Pohl’s The Coming of the Quantum Cats is a triumph of the imagination by a Hugo and Nebula–winning master of science fiction.
 
“A powerful novel of parallel worlds and might-have-beens, with an eerie, amazing finish!”—David Brin