Kim Cross, the author of a new true crime book, is not a true crime fan. “I don’t consume it and have never felt drawn to it as a writer,” she tells us early in “In Light of All Darkness: Inside the Polly Klaas Kidnapping and the Search for America’s Child.”

So why has she written a hefty nonfiction book about the 1993 kidnapping and murder of the Petaluma 12-year-old?

Her father-in-law, she writes, is a retired FBI agent who helped catch Polly’s killer. At her husband’s urging, she “did some research and realized the educational value and historical significance of this case.”

Words like these raise a reader’s expectations, and it’s true that this book, with its look at the crime’s influence on sentencing laws and law enforcement policy and procedure, is more substantial than some true crime content. But Cross isn’t the most judicious writer, hindering her account of an investigation that’s been chronicled many times before.

• 30 years later:Never-before-seen details revealed on the California kidnapping

The book, too, cannot always locate its emotional center; it’s dedicated to Polly and “those who tried to save her,” but the voices of the latter, who are accordingly concerned with the technical aspects of the investigation, are more prominent. Polly’s parents, sisters and best friend all declined to be interviewed.

On Oct. 1, 1993, Polly hosted two friends for a sleepover at her mother’s Petaluma home. About 10:30 p.m., a man later identified as Richard Allen Davis broke into the house. Armed with a knife, he tied up Polly’s friends and kidnapped Polly. He later strangled her.

The crime became national news. Not many people were online in 1993, so the story played out in print dailies, nightly newscasts and “America’s Most Wanted.”

The Idaho-based Cross aims to show “how relevant this thirty-year-old case remains today.” A non-consumer of the genre, she might be surprised to learn that lots of true crime books have similar goals.

Klaas investigators developed innovative ways of using fluorescent powder, over-the-counter glue and other items to obtain evidence, Cross writes. Their breakthroughs have “been used to train thousands of investigators in skills ranging from latent fingerprinting techniques to interview and interrogation methods.”

Some of what Cross relates is new; much isn’t. In 2003, the Chronicle reported that the case had fueled “stronger laws, better handling of child witnesses (and) a blueprint for responding to abductions.”

Surveying the impact of the “three strikes” law, enacted in response to Polly’s murder and intended to keep serial felons like Davis behind bars, she cites research finding that the statute contributed to mass incarceration and increased racial disparities in prisons. Her reporting is accurate but adds little to the discussion.

She’s better at explaining what law enforcement learned from the investigation. She interviews the former chief of the Lodi (San Joaquin County) police, who says that during a 1994 kidnapping investigation, his consultations with a Klaas investigator about command posts, media coverage and forensic artist sketches “played a huge part” in rescuing the victim.

And she notes that law enforcement agencies adopted policies and techniques shaped by the Klaas investigation. Today, for instance, the FBI wouldn’t subject child witnesses to polygraph tests like those that Polly’s friends underwent after the kidnapping.

But Cross’ narrative can be rather slack. She gives her sources room to tell self-serving, inessential anecdotes and includes pointless 30-year-old quotes, like one from a cop who says, “It is in the best interests of the investigation to maintain continuity in the investigation.”

Likewise, it’s interesting that Winona Ryder, who lived in Petaluma as a kid, helped draw media coverage to Polly’s kidnapping. But who cares that she dated Johnny Depp? Or that Depp tattooed Ryder’s name on his arm?

For its part, a late chapter would’ve been just as edifying without the jarringly specific details about the condition of Polly’s body when it was found in northern Sonoma County in December 1993.

Cross isn’t the first journalist to think she’s done important work, but her respectable, uneven true crime debut doesn’t rise above many other well-reported books that work the same territory.

Kevin Canfield is a freelance writer.