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I've been looking for this book for quite some time. It was in English. I remember reading it in high school, around 2005, and the copy I read looked old then, 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.
The method for traveling through worlds was kind of vague. For example he simply passes out after passing through a specific point in the Amazon when the first move takes place.
The one character name I am able to remember is User Annie. She doesn't get introduced until late in the book.