In S.P. Meek's 1931 novel The Drums of Tapajos American adventurers find a lost city (with an Atlantis link) in the Amazon rainforest. It's a flawed utopia with a dark side and the tale is interestingly ambiguous. Worth a read for lost world fans.
My review: https://vintagepopfictions.blogspot.com/2023/07/sp-meeks-drums-of-tapajos.html