This was quite an interesting book. It is plain fiction, though, despite Jules Verne's reputation for science fiction. It's not so much about the Amazon as it is about a man and then about the same man and his family—and a certain issue which I will explain (although it may spoil it for you; it is kind of a mystery story).
Anyway, the man eventually agrees to go with his wife on a trip down the Amazon, in Peru or Brazil (one was where they went and the other was where they were coming from). So, they create this giant raft (huge, quite), and go down the Amazon on it. They pick up a stranger on the way. Well, he's not quite strange as he looks familiar to one of them, and two of them had killed a monkey of sorts (which monkey had attempted to kill the man not long before), and the man expressed his gratitude for them having killed it, for it stole something quite valuable from him. At this point, you know the thing the monkey stole was a cryptogram (you know this from the beginning of the book), although you don't really know why this man is carrying it around and valuing it so.
Anyway, some of them become suspicious of the stranger, and they dislike him for some odd reason they can't fathom.
Now, I'm going to spoil some things.
After some time, the stranger approaches the said man I first talked about (I listened to it—so I don't know how to spell his name). Then, the truth comes out and we learn that the stranger wants to bribe the man for the marriage of his daughter in exchange for his vindication (he was once convicted of murder, and he escaped, fled the country and such, and this is why he was reluctant to go to the Amazon, earlier, since it is back in the country of his conviction). Anyway, we learn that he is really innocent, but that the stranger has proof of his innocence. Anyway, the man refuses, and the stranger turns him in. Apparently, the man wanted to come for the purpose of turning himself in for a retrial, but the stranger beat him to it, and so his case is less viable.
So, the man's son hears of this and gets really angry. He vows to kill the stranger. So, he and another go in search of the stranger. They have a dual (I guess is was legal to kill people this way, for there seemed to be no qualms about it when the authorities learned). The son wins and the stranger's body goes flying into the Amazon with the cryptogram (his son doesn't know about it, although the father had heard from the stranger that he had some sort of proof of his innocence, and he assumed it was in the form of a document). Anyway, the man has a very short time to prove his innocence before his execution or some such. He eventually convinces a judge of his innocence, although they still lack the proof. His son learns of the document and he and others go scouring the amazon for the body. After a long time, by something like a miracle (mixed with an attack from an electric eel; there is at least one other animal attack in the story), they find it and behold, it's a cryptogram. So, their hopes are dashed yet again (they are many times in this story) and they (especially the said judge) set in search of the key to the cryptogram.
I won't bother to tell the rest, as it's not necessary, but after it's all over, the author does wrap things up more than some authors do in some books—I quite enjoyed the resolution.
There are some things in the book that might be offensive, racially, to some, I've heard it said, although they might not be understood sayings to anyone outside of South America.
The writing style in some parts reminded me quite of Charles Dickens' in A Tale of Two Cities, however, the reading is much easier to follow (once you get used to everyone's names and who they are) and probably more exciting.
It isn't clear who the main character is for a while, although I trust this summary may help.
