Twenty Thousand Leagues Under The Sea by Jules Verne.
I have read it in high school and didn't have much appreciation for the premise/characters (especially Captain Nemo) surrounding the story until I reread it again as an adult.
Here, Nemo dominates the book. He provides a wonderful study of the complexities of humanity; Nemo is a man of his word, a genius, a gentleman (he is reduced to tears at the death of a crewman, he rescues an unknown pearl diver at his own risk)...yet there is something cruel and dangerous about him (he sinks ships and submerges beneath the waves to callously watch as they descend to their doom). He is an admirer of Abraham Lincoln and a self-proclaimed defender of the oppressed. He is the tyrant fighting tyranny...who evokes both sympathy and irony.
What I most enjoyed about reading 20K is that throughout the book, Verne infuses the idea of science through stunning descriptions of the undersea world...from the flora to its fauna and the curiosities of how the ocean operates.