Mystery novels set in Chicago are nothing unusual. But Chicago-based mysteries written by a non-Chicagoan who works as a librarian in Minnesota -- well, those are a little more unusual.

The librarian/author is Barbara Fister, a native of Madison, Wis., who has worked at the library at Gustavus Adolphus College in St. Peter, Minn., since 1987. "Through the Cracks" (Minotaur/St. Martin's, 320 pages, $25.99) is her third mystery, following "In the Wind" and "On Edge." All are worth reading; "Through the Cracks" is the best yet.

Fister's protagonist is Anni Koskinen, who spent 10 years as a Chicago police officer before being forced out because she testified against rogue colleagues. Koskinen is now a private investigator with a big heart, lots of courage and a penchant for danger.

Through a referral, she takes the case of Jill McKenzie, a sociology professor in rural Iowa. Twenty-three years earlier, McKenzie had been raped in Chicago and left for dead. The attack had received lots of publicity, but McKenzie remained anonymous to the general public and the rapist was never arrested. Researching the case so many years later, McKenzie realized the rapist continued to operate but police had failed to notice what looked like a pattern.

Koskinen says a reluctant yes to McKenzie, even while wondering if she can possibly solve a case so cold. After that, she becomes entangled in multiple story lines -- including an apparent wrongful conviction -- that might play out poorly in the care of a mystery writer less talented than Fister.

Fister is a master at plotting and pacing -- except in the final pages, where the action feels too compressed, and maybe downright rushed. Still, she ties the subplots together skillfully, leaving no questions unaddressed. The identity of the serial rapist might come as a surprise to most readers, but at least a few might pick up on Fister's subtle clues.

As in so many first-rate mystery novels, part of the fascination derives from the procedural aspects. Fister offers insights into the realms of police detectives, private investigators, prosecutors, defense lawyers, journalists, college professors, elected politicians, high-functioning autistics, priests, landlords and immigrants from Latin America.

In her acknowledgments section, Fister notes that the story derives from "the work of the Center on Wrongful Convictions at Northwestern University, and Adalberto United Methodist Church in Chicago, where immigration rights activists facing deportation have been given sanctuary." Fister employs her real-life inspirations well to create a believable fictional world that I hope she allows us to visit again.

Steve Weinberg is a Missouri-based writer.