I'm looking for a book I read as a young adult. My Dad loved it and he recently passed away. It was kind of comedic/irreverent (Terry Pratchett-like).
It was told in first person about this guy trapped in the multiverse/other worlds and couldn't get home. He had discovered that you could get to these other worlds and brought his best friend from childhood with him. His best friend turned into a villain and they had to fight each other?
It was almost told like a detective agency story---I think. One of the things I remember is the description that the journey to your destination is never the same as the journey back. That everything around you is different, even the time it takes to get there. Once he realized that, he was able to find a way to travel between worlds. I'm not sure if I'm getting that right since it was a long time ago.
I read it in 2001 but it might be much older since my dad got his books from used bookstores.