In the opening scene of Mona Simpson's sixth novel, "Casebook," 9-year-old narrator Miles Adler-Rich gets trapped under his parents' bed while planting a walkie-talkie. Seeking to glean the reasons why he's not allowed to watch "Survivor," he instead learns some unwelcome information: His father admits that he has a crush on another woman. It's a comical scene, Miles squirming with a full bladder, rolling from one side of the floor to the other as the underside of the bed dips and lifts, but soon Miles grows addicted to the thrill of spying on his family and is forced to parse the repercussions of invading others' privacy.

When his parents separate, Miles rummages through the mail, hides behind couches, and shimmies onto the roof to look for clues: Who cheated on whom? Whose idea was the divorce? Was it his fault? Did they blame his younger twin sisters?

As the novel follows Miles through middle and high school, his snooping grows more sophisticated. When his mother — a mathematician, "Small, with glasses, she was the kind of person you didn't notice" — starts dating Eli, an animal lover who works for the National Science Foundation in Washington D.C., and who flies in once a week to visit them in L.A., Miles grows suspicious of Eli's patchy biography. With the aid of his best friend Hector, Miles hooks up a wiretap, hacks into his mother's e-mail, and eventually, like Mayan in Simpson's second novel, "The Lost Father," hires a private investigator. But the gathered information rarely gives resolution to his central yearning: "I guess I want these things that seem weird to be — understandable." Miles' prowling comes in fits and starts, but when he learns Eli's big secret, he's forced to take action.

Simpson deftly avoids the nuisance of the overly precocious child narrator by pivoting the point of view to a Miles-in-hindsight, which allows her to freely oscillate between his often-funny adolescent spunk to wise observations that come only with time. It's a socially aware novel, too, full of timely questions about privacy, sexuality (Miles considers himself asexual) and our relationship to animals.

"Casebook" displays Simpson's signature impressionism. Think Seurat's pointillist dots rather than Van Gogh's lavish strokes. Miles' world is made of tiny scenes, images, anecdotes — collage-like but easy to follow. Fans of Simpson's "My Hollywood" will appreciate the thematic kinship, comedy, aphoristic observations and the unflinching look at the effects of divorce. "Casebook" tackles heavy themes, but with a touch lighter than her early novels — easier to digest, charming, sure-footed and as engaging as ever.

Josh Cook is editor at large of Minneapolis-based "Thirty Two Magazine."