Soon the police are questioning another wealthy old man, Conway Jefferson, who met the murdered woman at the hotel where she worked as a dance hostess. He explains that his son and daughter were killed in an accident years earlier and that he didn't want their spouses to have the burden of caring for him. Therefore, he says, "I got more and more fond of Ruby. I decided, gentlemen, to adopt her legally. She would become, by law, my daughter."
Miss Marple tells the police she believes Jefferson, and says she knew another old man, Mr. Harbottle, whose sister was called away to nurse a dying relative. In her absence, Harbottle began treating the maidservant as his companion. Miss Marple explains, "People said things, of course, but I believe there was no familiarity of any kind. It was simply that the old man found it much pleasanter to have a young, cheerful girl telling him how clever and amusing he was than to have his sister continually pointing out his faults . . . ."
Not content to give one example, Miss Marple tells a similar story about a "Mr. Badger" and goes on to cite the legend of King Cophetua and the beggar maid. Near the end of the book, a butler says he's seen many older men "adopt" younger women.
Christie's reason for piling up all these examples becomes clear when Mrs. Bantry begs Miss Marple to expose the murderer. Otherwise, she says, people will always assume her husband had an affair and killed to cover it up. Her husband will be shunned, will become lonely and depressed, and will die too soon. False assumptions are deadly.
I admire Christie for choosing to explore a relationship that is usually assumed to be sexual and for coaxing the reader along toward believing it isn't. The Body in the Library gives us a puzzle---How did the body get into the library? Who wanted it there? And why?---and a solution. It also delivers a powerful theme.