Twenty Thousand Leagues Under the Sea is a classic science fiction novel that follows the story of an expedition, whose members include Pierre Arronax and Ned Land, tracking what they believe to be a sea monster. The "monster" turns out to be an advanced submarine created by Captain Nemo, a man with a hatred for society. Once they have uncovered his secret, Captain Nemo refuses to let the expedition members leave his mysterious submarine, and together they explore many parts of the sea until Nemo's folly allows them to escape.
Boasting the largest collection of book summaries, BookRags is the best option for titles you can't find elsewhere. They offer all of the basics (quotes, chapter summary, characters, historical context, literary criticism) but also walk through a few major topics that recur throughout the book, which can be particularly useful for essay writing.
Their book summaries aren't as good as other providers, but PinkMonkey often has interesting insights that aren't mentioned elsewhere on the Internet. For example, they go over literary elements (like setting, conflict, and mood), compare and contrast, and symbolism/motifs. It's a good site if you want to find a unique fact or angle that other students don't know about.
If you're bored by traditional book notes, try Shmoop! They write their summaries in an approachable, entertaining style. Their comprehensive offering includes chapter by chapter summaries, themes, quotes, character analysis, study quizzes, and even flash cards and infographics.
Includes the standard sections found in most book notes, including a book synopsis, chapter by chapter analysis, author biography, character list, essay topics, book report ideas, and study questions. This website is less comprehensive than other book notes sites, but could be useful if you're looking for a quick review.