"I'm asking you to marry me, you little fool" is a quotation from which novel?

"Rebecca" is a 1938 Gothic novel written by Daphne du Maurier (13 May 1907 – 19 April 1989). The novel depicts an unnamed young woman -- the narrative voice in the story -- who impetuously marries a wealthy widower, before discovering that both he and his household are haunted by the memory of his late first wife, the eponymous Rebecca.

A bestseller which has never gone out of print, "Rebecca" sold 2.8 million copies between its publication in 1938 and 1965. It has been adapted numerous times for stage and screen, including a 1939 play by du Maurier herself, the film "Rebecca" (1940), directed by Alfred Hitchcock, which won the Academy Award for Best Picture, and the 2020 remake directed by Ben Wheatley for Netflix.

Many people remember the story because of the sinister housekeeper Mrs. Danvers, or the Cornwall estate Manderley, or the novel's famous opening line, "Last night I dreamt I went to Manderley again."

The quotation in the question title is another of the memorable "Rebecca" classics. Thrown together by chance in a Monte Carlo hotel, the narrator (who is there as paid companion to a rich, elderly American woman) and Maxim de Winter, the wealthy English widower, are thrown together by chance. They get to know each other (without the benefit of the rich American woman's company). After two weeks, Max de Winter makes his move. The young woman thinks she is being offered employment as a secretary. He clarifies with this crisp “little fool” one-liner.