Vladimir Nabokov's novel 'ADA' is a three-letter word in the crossword. It was published in 1969 and is set in the 19th century on a fictional island in the Baltic Sea. The novel follows the story of two cousins, ADA and Van Veen, and their complicated relationship. Nabokov's other novel, 'Pnin', is a four-letter word in the crossword. It was published in 1957 and follows the story of a Russian professor living in America. The novel explores themes of identity, displacement, and alienation.