With only four novels in 14 years, April Smith cannot be regarded as a prolific author of crime fiction. Read them, however, and it is not difficult to tell why (aside from the usual stuff of a marriage, children, working as a journalist whose articles have appeared in such disparate venues as Mademoiselle, Antioch Review, the Los Angeles Times, Rolling Stone, and Atlantic Monthly, as well as writing eight made-for-television movies, producing “Chicago Hope,” “Cagney and Lacey,” and being the executive consultant on “Lou Grant,” picking up three Emmy nominations along the way).

No, the real reason why she has written so few books is that her novels are so complex. They are not dashed off. They are designed to baffle her many readers and they succeed. Take her most recent novel, “Judas Horse” (Knopf, 318 pages, $23.95), which is just slightly more complicated than assembling a gas grill. Trying to follow exactly what is happening, and who is doing what to whom, is not unlike attempting to explain to a foreign visitor why West 12th Street intersects with West 4th Street.

The heroine is Ana Grey, who was also the star of two previous books by Ms. Smith. In the first, the justly lauded “North of Montana” (which refers to the nouveau riche section of Santa Monica, not Big Sky country), the young FBI agent solves an apparently “impossible” bank robbery and seems ready to have an accelerated career path until thwarted by an old-boy network.

Proving that attractive and tough young female law enforcers aren’t that different from their male counterparts, Ana becomes involved with a cool and compassionate police detective, Andrew Berringer, but also starts to develop inappropriate feelings toward her partner, Mike Donnato, who happens to be married.

Her second novel, “Be the One,” was one of the best baseball mysteries ever written, and featured a different protagonist, Cassidy Sanderson, a baseball scout — and if you think there is an old-boy network in the FBI, just imagine what an easy time a female competing with guys to sign Dominican players will have.

In the second Ana Grey novel, “Good Morning, Killer,” her major case involves the kidnapping of teenage girls. By this time, she has become a bit obsessive about Berringer, but hasn’t abandoned her nasty thoughts about Donnato.

Apparently never assigned to predictable cases, Ana gets a humdinger in “Judas Horse,” when she has to go undercover to stop and catch extremists in FAN, an animal rights group that has turned to violence.

Still recovering from the trauma of having shot and killed a berserk policeman, Ana has been seeing a therapist who decides that she’s ready to go back to work. Almost immediately asked to go undercover in spite of her fragile mental state, she is sent to the FBI’s special school. If you don’t have a stout heart, skip this section (spoiler: she passes) because the incidents with which she must deal are so fast, harrowing, and breathtaking that they are like skiing down the expert slope while juggling vials of nitroglycerine.

She is able to infiltrate the group, which is responsible for the death of another FBI agent with whom she had an affair (okay, little Ana gets around). The bureau warns her about Stockholm syndrome, and, sure enough, the adumbration is sensitive and wise, as there are moments when Ana’s sympathies need to be questioned.

“Judas Horse” is not so much a cop novel as a thriller, and it lives up to that appellation. I am fearful of giving away too much, but mystery readers should never be surprised to learn that everything they thought they knew for a couple of hundred pages is not always what it seems, and that goes triple in this runaway but cagey novel that never lets up.

A fundamental understanding of any law enforcement operative who goes undercover is that some days you’re the arrow and others you’re the target. Everything depends upon your own wits and skills in effectively earning the trust of the bad guys while continuing to trust your colleagues, and having them maintain their trust in you, too.

What do you do when you learn that maybe everyone you trust can’t be trusted? And, worse, when you don’t know who, specifically, can or can’t be trusted. While, in the meantime, even those who can be trusted begin to wonder if they can trust you.

Like Ana, you are in a pickle.

Mr. Penzler is the proprietor of the Mysterious Bookshop in Manhattan and the series editor of the annual Best American Mystery Stories. He can be reached at ottopenzler@mysteriousbookshop.com.