Fantasy book set in Multiple settings

I've been looking for this book for quite some time. I remember reading it in high school but the name of it is escaping me. It's about a man in the British army if I'm remembering correctly, searching for his missing brother in the Amazon. He ends up traveling through several different worlds throughout the book. There are only sporadic details that I remember other than that, such as the main character being in a card game on the trip to the Amazon where the other players are cheating and one of the main character's companions having a revolver with a design on the grip.