Ada, or Ardor: A Family Chronicle, by Vladimir Nabokov, was published in 1969. It tells the story of two lovers, Van and Ada Veen, who meet as teenagers, believing they are cousins; they later find out they share the same mother and father. It takes place in the late 19th century in the imaginary Antiterra - a kind of alternative Earth. Upon its publication, The New York Times called it "a love story, an erotic masterpiece, a philosophical investigation into the nature of time."

Read more about Ada:

• New York Times review
• Wikipedia page
• The geography of Antiterra
• The full text of Ada, with extensive annotations