As James Gunn’s highly anticipated “Superman” soars into theaters this month, fans can journey into the DC Universe with a new book about the making of the film. Penned by veteran film author James Field,

“It was just difficult to figure out where I was going to come from with Superman ,” Gunn said speaking to io9 in Los Angeles last week. “And I think that it really started with my dog in a lot of ways. The idea of putting in Krypto, I think, was the fun part for me, because that was something that we hadn’t really seen in a Superman movie.”

James Gunn tells PEOPLE about how his rescue dog Ozu inspired the new Superman movie, specifically the superhero's canine, a fluffy, white dog named Krypto