If Spenser withholds evidence from the FBI, an agent warns him, he will face “the full force” of the United States government. To this, the Boston private eye says, “Eeek.”

“Now & Then” (Putnam, $25.95), the 34th book in a series that began with the publication of “The Godwulf Manuscript” in 1973, finds Spenser as feisty and smart-alecky as ever. Despite a quarter century of getting shot at, and occasionally actually shot, both he and his friends are aging gracefully.

Susan, the love of his life, is in her 50s now, but she can still turn the head of a young thug now and then. Spenser and his inscrutable sidekick, Hawk, must be 60, but after years working out on the heavy bag, they remain the pair you’d most want by your side in a fight – as the villain of the tale learns the hard way:

“He turned back toward me and I hit him with a left hook. It was the left hook I’d been working on with Hawk for years. The left hook that if I’d had it as a kid Joe Walcott would never have beaten me. The left hook I’d been saving for a special occasion. It was a lollapalooza. I felt all of me go into the hook. I felt it up my arm and into my chest and shoulder and back. I felt it in my soul.”

Robert B. Parker got the Spenser saga off to a strong start with the first 10 novels, but then the series faltered. Some of the books suffered from thin plots, and the wisecracking detective sometimes turned into a parody of himself. In recent years, only occasional Spenser novels, including “School Days” in 2005, approached the quality of the writer’s earlier work.

“Now & Then” is a return to form – one of the better Spenser novel in 20 years. The plot is unpredictable, the characters are richly drawn and the dialogue crackles. Although Parker’s work rarely approaches the excellence of more literary crime writers such as George Pelecanos and James Lee Burke, his best books deliver a quick, entertaining read in a crisp, unadorned style.

As the story begins, a nervous man enters Parker’s office and says his wife has been keeping strange hours lately. He asks the detective to find out what she’s been up to. Soon, what appeared to be a simple divorce case has Spenser chasing a shadowy international terrorist who has killed at least two people.

Susan, Spenser’s psychologist-girl friend, is a major presence in this book, and that’s rarely good news. Usually, she comes off as insufferably precious as she engages her big, bad boyfriend in pages of boring, meaningless psychobabble about his choice of profession, his use of violence or their annoyingly cutesy-pie relationship.

This time, however, Susan is integral to the plot. Better yet, most of the psychobabble is gone, replaced with real, human conversation that deepens our understanding of why this unlikely couple has stayed together for so long.