A bit past the midpoint of the fourth Cormoran Strike mystery — at 650 pages the “Moby-Dick” of the series — the one-legged private investigator and his devoted female assistant, Robin, are looking at eight suspects for the murder of a government minister. That loosely translates into about 60 pages per suspect, not counting the also-rans, which tells you that “Lethal White” — written by J.K. Rowling under her pen name Robert Galbraith — likely tells us more about each of the potential killers than they know themselves.

That’s certainly true of Billy Knight, a sad, lost stranger suffering from mental illness who gets the story rolling by bursting into Strike’s office and claiming he witnessed a child being strangled and buried years ago. He claims the killer was his older brother Jimmy, a rabid anti-Semite who travels under the cover of a socialist organization.

Then there’s Raphael “Raff” Chiswell, the black sheep of his family, who while high on drugs during college ran over a young mother with his car. He is the son of Minister of Culture Jasper Chiswell (pronounced “Chizzle”), from whom Jimmy Knight is trying to extract 40,000 pounds in hush money for something, Chiswell says, “I would not wish to share with the gentlemen of the fourth estate.”

Connections, connections: The ties between the suspects of the Parliament murder keep multiplying. You can bet that the child killing and other premature deaths are connected to it. (“Lethal White” is a term used for a white foal born with a defective bowel that, like a baby in the book, is unable to survive.)

Having lost his leg to an IED in Afghanistan, Strike has an increasingly difficult time hustling about on his prosthetic limb — not the greatest thing to have when people are beating you up or you’re chasing a suspect on foot.

Robin, who suffers post-traumatic stress disorder following the brutal attack on her in “Career of Evil,” also has to contend with her loveless marriage to the only man she has ever been with. Working again with Strike, now famous for catching the Shacklewell Ripper, is good therapy. Her boss had fired her following a bad falling out. But the more she works, the more she has to lie to her husband, Matthew, who disdains the job and her employer and, like too many people in her life, thinks he knows what’s best for her.

Dedicated readers of the series will want to know that, yes, Strike and Robin still moon after each other. Strike, who had come this close to having her in his arms before regrettable X factors pushed her into matrimony, tries to forget his troubles in the arms of another woman. But if ever there were a character written in disappearing ink, it is Lorelei Bevan, who, Strike muses, “played Aphrodite to his Hephaestus.”

As ever, the byplay between Galbraith’s classic, Agatha Christie-inspired plotting and flighty characters on the one hand and such contemporary details as the rampant use of the F-word, text messages and Kanye West creates an enjoyable floating time feel. With its subtle treatment of politics, class warfare and displacement, this is a book that essentially could be set at any time during the past hundred years. But while the complicated plot is admirably well-constructed, “Lethal White” lacks the narrative juice of past installments in the series. For all its twists and turns, you never really get caught up in the mystery, which never seems to matter as much as the star-crossed feelings that Strike and Robin have for each other.

There is a wide assortment of interesting, assertive women in the book, including Chiswell’s wonderfully irate wife, Kinvara (whose name is common compared with such Chiswells as Fizzy, Flopsy and Pong) and Strike’s straight-laced half-sister, Lucy, with whom he shares a difficult childhood. But a powerful mini-monologue directed at Strike about men and crime seems out of place: “Ultimate responsibility always lies with the woman, who should have stopped it, who should have acted, who must have known. Your failings are really our failings, aren’t they?”

Robin reveals herself to be quite good at skullduggery in the halls of power and at the art of disguise (love the chalked hair). But having seen her make such striking personal advances in “Career of Evil,” it’s disappointing to see her take two steps back here. She spends way too much time rationalizing her bad marriage. And on the job, Strike is the one who comes up with all the big insights.

“It became impossible for me to remain an idle spectator any longer,” reads one of the excerpts from Henrik Ibsen’s play “Romersholm” that precede each chapter of “Lethal White.” Here’s hoping that in her next adventure, Robin leaves behind idle-hood in all good ways.

Lloyd Sachs, a freelancer, regularly reviews crime fiction for the Tribune.