"The Wrecks of Time" is a 1966 novel by Michael Moorcock, published as half of an Ace Double. It was apparently also released in an unedited version, several years later, but I have only read the Ace version.
Michael Moorcock is most famous for the Elric saga, a sprawling sword and sorcery epic. This book, however, was science fiction, although it does retain a certain dreamy quality. The book follows one Professor Faustaff, a man who has discovered that there are multiple earths coexisting with our own, and has also discovered a way to travel between the fifteen of these alternative earths. This ability quickly comes in useful, as he finds that the alternative worlds are being eroded, their physical geography altering and people's memories forgetting what the world was once like. For example, at the introduction, Faustaff is in Earth-3, where the only country is the United States, a large circular desert with Los Angeles as its capital, in the center of the country. Faustaff is the leader of an organization that assists the people in the eroding earth, including by evacuating them. For adversaries, he has two groups: The Scavengers, former colleagues of his that loot the eroding dimensions, and the "D-Squads" who seem to be responsible for the destruction of dimensions.
Does all of that seem hard to keep track of? It was hard for me to understand the set-up as well, especially as the action starts. Faustaff is a pacifist, and the action of the book mostly consists of him getting kidnapped or interdicted by various factions and mysterious figures, who give him mysterious, portentous speeches as the universe breaks down. Through a series of increasingly unusual events, we are eventually given a finale that shows what would become a staple of Moorcock's later work: virtual gods who, despite their vast power, are strangely placid and vulnerable.
Despite some flaws, I think this is a great early work of a famous science-fiction/fantasy writer, and that one aspect of the Ace format, the zealous editing, might have made it easier to read.