There is something profound in the literary trope of the odd couple. It may be the cliché that has launched innumerable forgettable buddy comedies and cop dramas, but the notion that two drastically dissimilar people—Holmes and Watson; Harold and Maude; Oscar Madison and Felix Unger—can value each other, and even function as a unit, implies an inspiring faith in human affection, loyalty and understanding.

That underlying resonance just adds to the appeal of Will Thomas’s marvelous mystery novels, which center on an oddball pair of Victorian sleuths—or, as they prefer to be called, “enquiry agents”—named Cyrus Barker and Thomas Llewelyn. Intensely colorful and atmospheric, filled with remarkably vivid (and sometimes eccentric) characters, and distinguished above all by a vision of a very cosmopolitan, ethnically diverse Victorian London, the series will receive a long-awaited new installment with the upcoming publication of “Fatal Enquiry,” scheduled for May (Minotaur Books).

The series kicked off a decade ago with the publication of “Some Danger Involved,” a high-stakes whodunit that introduced Barker, a former sea captain whose expertise includes encyclopedic knowledge of the Orient, as well as of London, and an intimidating command of weaponry and the martial arts. When he hires the endearing young Oxford University dropout Llewelyn as an assistant, the two make an improbable team. Barker is a tough, hard-headed, multilingual world-traveler whose demeanor tends toward terse intensity; Llewelyn—the narrator of the books—is an amiable, callow, novel-reading romantic who has survived a tragic past with a smart-alecky sense of humor.

Acting as consultants and rivals to Scotland Yard, maneuvering through idiosyncratic nooks of the Victorian underworld, and running up periodically against the realities of the British Empire, the pair bear some resemblance to Holmes and Watson. And yet, you would never see Holmes and Watson heading to church of a Sunday, as Barker and Llewelyn do. Barker turns out to be a dedicated Baptist, given to attending Charles Haddon Spurgeon’s Metropolitan Tabernacle. After signing on as Barker’s assistant, Llewelyn (a Methodist, by upbringing) begins attending on Sundays as well.

It is implied, though by no means stated outright, that Barker’s faith has increased his openness to other cultures. After all, this is a fellow whose favorite London hangout is an Chinese restaurant that serves only authentic Chinese fare and is accessible only by a long tunnel that runs under the Thames. Barker is a longstanding friend of the owner, a mysterious tattooed figure named Ho.

The world of Londoners of Chinese heritage dominates one of the Barker & Llewelyn books, “The Limehouse Text,” in which the enquiry agents are caught up in international skullduggery involving a book stolen from a Nanking monastery. But that adventure does not constitute the series’s only plunge into a subculture of Victorian Britain’s capital city: “Some Danger Involved” explores the lives of Jewish Londoners. (During the course of the book, the surprised Llewelyn learns that Barker employs a Jewish butler, a young man who becomes one of several quirky, multidimensional recurring characters in the series.) “To Kingdom Come” deals with Irish nationalists plotting insurrection against British rule. “The Black Hand” concerns Italian expatriates and organized crime syndicates. And—it turns out, in one of the series’s long-running comic subplots—the temperamental chef of the best French restaurant in Soho just happens to moonlight, in the mornings, as Barker’s household cook.

Breezing from one section of the Thames-straddling global village to another is all in a day’s work for the indefatigable Barker. “It seems strange to find a Baptist such as yourself so well acquainted with Jewish customs,” Llewelyn hazards after his employer has discoursed knowledgeably on the all-in-Hebrew funeral service the two sleuths have just attended in “Some Danger Involved.”

“Yes, well, knowledge is a good thing,” Barker replies.

Of course, knowledge can be a frightening thing, too: That’s a truth that a sleuth inevitably runs up against. In “The Hellfire Conspiracy”—in which our odd-couple team of enquiry agents tracks down those responsible for a run of heinous child kidnappings and murders—Llewelyn is taken aback by the cruelty of the evildoers they are tracking.

“I thought this was a Christian country,” Llewelyn broods aloud.

A churchgoing detective like Barker knows better. “Then you are misinformed,” he replies to his assistant. “We live on a mean, sinful planet, Thomas, and it shall only get worse if the Lord should tarry.”

In a post on the Barker & Llewelyn Facebook page a few months ago, author Thomas suggested that G.K. Chesterton’s Father Brown stories have been something of a touchstone for him. The Father Brown canon “disproves the notion that a detective must be a passionless scientist and agnostic,” Thomas wrote. “I hope Barker is doing the same.”