Interesting novel by Tracy Chevalier! Girl With A Pearl Earring is a painting by Johannes Vermeer, a 17th Century Dutch painter. In the novel, Chevalier hypothesizes that Vermeer's servant girl, Griet, is the girl in the painting.
In the story, we find that Griet's father has been blinded in an accident, and her parents are forced to rely on Griet's wages to sustain the family. Working in Vermeer's household poses myriad challenges for Griet: she must contend with Catharina, Vermeer's pregnant wife, who dislikes her on sight. The household also includes Maria Thins, Catharina's formidable mother, five children (Maertge, Lisbeth, Cornelia, Aleydis, and baby Johannes) and the family housekeeper, Tanneke. Griet must be careful lest she incur the jealousy of Tanneke and she must also contend with Cornelia's malice towards her.
Emotionally, Griet struggles with her own fascination with Vermeer; at one point, she is even disappointed that Vermeer does not reveal to Catharina that she, Griet, has been working with Vermeer closely on his paintings. We get the impression that Vermeer manipulates Griet for his own ends as well, sending her to run errands for him, allowing her to grind and mix pigments for his work and also leading her to think that her opinions are important to him in regard to his paintings. Even though Griet works in Vermeer's household out of a sense of duty (her destitute family are dependent on her wages), Vermeer also does whatever he can to sustain his own income for the sake of his growing family. He protects Griet from the demand to paint Van Ruijven and Griet together. The last maid painted with Van Ruijven is eventually found to be pregnant and is not heard from again in polite society. Vermeer recognizes the importance of Griet in appeasing the licentious Van Ruijven but he refuses to give up Griet so easily to Van Ruijven's lusts; his growing family compels him to find a solution. In the end, Vermeer finds an artful compromise, although it is one that will enrage his wife, Catharina. He paints Griet wearing Catharina's prized pearl earrings.
Vermeer is willing to test the boundaries of Catharina's patience in order to finish the commission he has been given. The famous painting is born and Vermeer manages to financially sustain his household for a brief period. Later on, we find that Vermeer's family is in debt and Griet has married Pieter to maintain her own family's survival even though she initially rejects Pieter because she detests the bloody aprons Pieter wears in his work.
So, you can see that both Vermeer and Griet do what duty tells them to: Griet marries a man she might never have accepted in other circumstances and works in a capacity she might never have entertained, all in order to prevent her family from falling into deep poverty. Vermeer manipulates Griet for his own aims in order to do the same: he does not really love her, although Griet might have fancied his attraction to be real. Indeed, it is Van Leeuwenhoek, Vermeer's friend, who warns Griet that Vermeer is only really interested in paintings and that his obsession with paintings takes precedence over his interest in people. Griet realizes this and that may be why she accepts Pieter's proposal; it is a far safer way to fulfill her duty to her family through marriage to Pieter, whatever her thoughts may be about his bloody aprons.