The Wild Duck looks at the tragedy that can occur when a person is forced to confront the truth. After Gregers Werle returns home from an exile, he learns that his father, Hakon, had an affair with the wife of Hjalmar Ekdal, a former schoolmate of Gregers. Gregers is convinced than Hjalmar needs to know about his wife's past behavior, revealing to him that Hedvig, the couple's daughter, might not actually belong to Hjalmar. Rejected by the man she thought was her father, Hedvig ends up committing suicide.
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.
The original provider of professionally-written study guides, Cliffs Notes were available in print form before the Internet even existed. Their content is very comprehensive and includes an analysis of characters, detailed chapter by chapter summaries, critical essays, essay topics, and many other sections.
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.