If you wish to join the cult of admirers of Alexander McCall Smith, there are two things that you must know in advance. First, Mr. Smith’s novels are delightful, often making you laugh out loud. Second, they appear in series, and you must read each book in a series in the order of its appearance to be fully informed about the characters and their milieu.

The six books in Mr. Smith’s first and best-known series – the “No. 1 Ladies’ Detective Agency” – detail the adventures of the wonderful Precious Ramotswe, who is the first and only female private detective in Botswana. His second series, “Portuguese Irregular Verbs” (actually written and published privately before the “No. 1” books), is a hilarious spoof of academic life as seen through the misadventures of Dr. Moritz-Maria Von Igelfeld, a pompous German philologist. The third series is called “The Sunday Philosophy Club,” and its hero is Isabel Dalhousie, an elegant and keenly intelligent Edinburgh gentlewoman who is the part-time editor of a philosophy journal called the Review of Applied Ethics.

“Friends, Lovers, Chocolate” (Pantheon, 272 pages, $21.95) is the second in the “Sunday Philosophy Club” series. Not unlike Agatha Christie’s Jane Marple, Isabel Dalhousie has a penchant for getting involved in mysteries in need of a solution. Where others might turn away and say it was none of their business, Isabel cannot help getting involved. It is part of her vocation as a philosopher, she reasons; she has a moral obligation not to turn away when she thinks she is needed. Her daily life, from actions to trivial decisions, is an ongoing exercise in reasoning and applied ethics.

In the first book, Isabel becomes involved when she sees a young man plunge to his death from the top row of seats at the opera house: Was it an accident or murder? In this book, she becomes involved when she meets a man who asks for her help. After receiving a heart transplant, he periodically experiences a jolt of pain followed by a vision of a face. The heart implanted in him came from a young man, and he wonders whether he had seen the face of someone who had caused that man’s death. Could the heart remember? It becomes Isabel’s mission to find out whose heart he had received and the circumstances of the young man’s death.

Like all of Mr. Smith’s books, this one has a subtext that goes far beyond the plot. Mr. Smith is concerned with moral dilemmas, and he dramatizes them by showing how people cope with them in everyday situations. Given her philosophical training, Isabel is constantly engaged in moral reflection. Her observations about daily life are rife with judgments: She looks at two portraits of men she knows, one showing pride, the other an innate goodness. As she gazes, she thinks:

In an earlier age, it might have been possible to believe that goodness would prevail over pride, but not any more. The proud man could be proud with impunity, because there was nobody to contradict him in his pride and because narcissism was no longer considered a vice. That was what the whole cult of celebrity was about, she thought; and we feted these people and fed their vanity.

The characters in Mr. Smith’s novels keenly experience a sense of loss, usually associated with the erosion of cultural or national identity by the relentless forces of modernization. Precious Ramotswe clings stubbornly to traditional Botswanan ways of behaving civilly towards others, knowing that they are threatened by the new, callous ways. A character in “Friends, Lovers, and Chocolates” complains bitterly when he cannot buy a traditional triangular Scottish oatcake.

“You may think it’s ridiculous,” he says, “but it’s just that there are so few things in this world which are authentic. Local. Little things – like the shape of oatcakes – are very important. It’s nice to have these familiar things about one. There are so many people who want to make things the same. They want to take our Scottish things away from us.”

Isabel Dalhousie tries to maintain a sense of moral order and personal responsibility in her own small universe. She treats everyone with equal courtesy, which makes her “profoundly egalitarian,” but she is uncomfortable with “moral relativists and their penchant for non-judgmentalism. But of course we must be judgmental, she said, when there is something to be judged.” In her capacity as editor of a journal, she rejects a treatise titled “The Rightness of Vice,” which argues that people should feel free to do whatever they really wanted to do. She simply could not defend the indefensible.

In small ways, Isabel’s (and Alexander McCall Smith’s) judgments about the importance of order and civility are the scaffolding for the book. Isabel meets an acquaintance for lunch at a restaurant, and while conversing, she “moved her fork and knife slightly so that they were parallel with each other; a small detail, perhaps, but that was what zero tolerance was all about. One started with the cutlery.”

The plot in an Alexander McCall Smith novel is a convenient fiction around which Mr. Smith can build his own moral universe. Grace, Isabel’s housekeeper, is a one-woman Greek chorus (or Scottish chorus) of moral truth. When Isabel is considering a paper about “the ethics of the buffet bar,” and whether it is ethical to take extra food for the next day’s picnic, Grace says, “You shouldn’t take what’s not yours. Is there anything more you can say about it?”

Whether he is writing about Botswana or Scotland, Mr. Smith has a deep affection for the place and its denizens. Where Mr. Smith ventures, the reader feels at home. I loved meeting the wonderful Precious Ramotswe; I roared at the highbrow foolishness of Dr. Igelfeld; and I admired Isabel Dalhousie’s tenacious hold on a life of culture and moral commitment. What more can one ask from a writer?

Ms. Ravitch is author of “The Language Police” and “Left Back: A Century of Battles Over School Reform.”