Jules Verne 
Eight Hundred Leagues on the Amazon 

Joam Garral grants his daughter’s wish to travel to Belem where she wants to marry Manuel Valdez in the presence of Manuel’s invalid mother. The Garrals travel down the Amazon River using a giant timber raft. At Belem, Joam plans to restore his good name, as he is still wanted in Brazil for a crime he did not perpetrate. A scoundrel named Torres offers Joam absolute proof of Joam’s innocence but the price that Torres wants for this information is to marry Joam’s daughter, which is inconceivable to Joam. The proof lies in an encrypted letter that will exonerate Garral. When Torres is killed, the Garral family must race to decode the letter before Joam is executed… (Excerpt from Wikipedia)