The Pilgrim's Progress is a 1678 Christian allegory by John Bunyan. The work is divided into two parts, telling the stories of two journeys from the "City of Destruction" to the "Celestial City." In the first book, Christian is burdened by the knowledge of sin after reading the bible. He leaves his family behind, traveling to the Celestial City. Along the way he meets Evangelist, Jesus (as Good-Will) and Hopeful as he struggles through numerous temptations and challenges. In book two, the rest of Christian's family completes a similar journey.
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.
You won't get any frills with GradeSaver -- just the content you expect. Their study guides include a full plot summary, themes and character analysis, study questions, and the e-text of the book. One hidden gem is their collection of literature essays, but you'll have to pay for it.