Both a family saga and the story of one man's struggle to discover his identity, Middlesex tells the story of Calliope Stephanides, or Cal, an intersex man who was raised a girl, only to learn in his teens that he is a male. Due to a history of inbreeding in his family, Cal has a rare genetic condition. He ultimately decides to live as a male, despite having spent years as a female. The conflict between nature and nurture and between choice and fate are major themes in the book.
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.
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.