Gurps Infinite Worlds by Kenneth Hite

A comprehensive role-playing sourcebook that presents a multiverse framework for running campaigns across alternate histories and parallel Earths, offering rules and equipment for interdimensional travel, an organization to police or exploit cross-world activity, dozens of fully sketched alternate worlds and cultures, guidance for creating and balancing divergent technologies and magics, and plentiful adventure seeds and campaign hooks to support exploration, espionage, and large-scale conflicts among realities.