WASHINGTON – The wildly popular PBS drama, “Downton Abbey,” wraps-up its third season on Sunday.

Eager fans will tune-in to learn the fate of the Crawley family and the Downton estate — and undoubtedly to witness the witty deliveries of the Dowager Countess of Grantham, played by Maggie Smith.

The British drama first aired in the U.S. as a part of “Masterpiece Classics” in 2011, but what many fans don’t know is that the Emmy-winning show was inspired from a book published in 1989.

“To Marry an English Lord,” by Gail MacColl and Carol Wallace, details the time period when American heiresses would travel to Britain and swap money for titles, much like the character Cora Crawley in “Downton Abbey.”

Author Carol Wallace told WTOP’s Dean Lane that she was astonished when she saw an interview with “Downton Abbey’s” screenwriter, Julian Fellows. In the interview, Wallace learned that prior to creating “Downton Abbey” the producer read “To Marry an English Lord.”

Wallace says that “Downton Abbey” picks-up where “To Marry an English Lord” leaves off — it tells the story of what happens after an American girl marries an English aristocrat.

In “Downton Abbey,” the American girl is Cora Crawley, Countess of Grantham, and the English aristocrat is Robert Crawley, Earl of Grantham. Although the show never gives too much background into how Cora and Robert met, it does touch on the fact that Robert married Cora for her money, which he needed at the time to save Downton.

“American women would go to London in the 1880s and 90s and be presented to young men at parties and dances and balls,” explains Wallace. “That’s how they found their husbands.”

Of course, the drama of “Downton” involves more than just the marriage of Lord and Lady Grantham. The stories of love, scandal and social and political struggle are interwoven in an extensive list of characters, including other members of the Grantham family and the staff at the Downton estate.

Fans of “Downton Abbey” are familiar with the Crawley’s overflowing concern and respect for their household staff. However, Wallace explains that sensitivity toward servants was not always common among American women who married into English traditions.

“For American women, the hierarchy of English servants was very different,” Wallace says. “To have to respect that professional hierarchy was a big learning curve for Americans.”

For viewers, this is especially evident when Cora’s American mother comes to visit Downton in season three. Throughout her visit she rejects the notion of “outdated” English traditions, and even suggests dinner guests “picnic” instead of sit for a formal dinner.

While the show’s producers remain tight-lipped about how the season will close, critics guarantee that Sunday’s finale will end with a bang. “Downton Abbey” airs on PBS at 9 p.m.

WTOP’s Rachel Nania and Dean Lane contributed to this report. Follow @WTOP on Twitter.

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