Books like Tad Williams' Mirror World: An Illustrated Novel

Mirror World is a story of disaster and survival. Just before the millennium (1999), gigantic mirrors appear in the world's major cities, bisecting streets, parks, and buildings. They are as beautiful as they are mysterious, until their terrible purpose becomes clear. They are portals, through which predatory insectile aliens swarm, killing or carrying off everything in their path. Humankind's only chance is to attack the "bugs" at their source - in Mirror World. Sixty volunteers are bioengineered to become human weapons. Some of the biotroopers pursue the Bugs through further portals - into the dark heart of the Universe. Others remain on Mirror World, setting up city states such as Shades and Looking Glass, creating a second Earth with new wonders and new evils all its own.