Permutation City by Greg Egan

Permutation City is a 1994 science fiction novel by Greg Egan that explores many concepts, including quantum ontology, via various philosophical aspects of artificial life and simulated reality. It won the John W. Campbell Award for the best science-fiction novel of the year in 1995 and was nominated for the Philip K. Dick Award that same year. The novel was also cited in a 2003 Scientific American article on multiverses by Max Tegmark.