Loneliness is central to Sue Grafton's R Is for Ricochet, the 18th novel to feature my favorite American gumshoe, California's Kinsey Millhone. Kinsey, a twice-divorced, slightly shy, somewhat tough investigator, was 32 when this series began in 1982; now she's 37, and the books are stuck in the late '80s, which means we're just reaching the inevitable: a plot involving computer disks. It may be a rule of the field that there has never been a really great mystery whose climax involves a computer, and ''Ricochet'' is no exception, but fans will be happy to see that at least Kinsey picks up a hot boyfriend along the way.
This isn't one of Grafton's stronger installments, partly because her woman-scorned plot, in which Kinsey is assigned to handhold a recent parolee whose boyfriend/boss screwed her over, forces her to be, as she admits, ''a minor character in someone else's play.'' Watching Kinsey on the fringe is oddly sad; Grafton, a sharp and observant writer, doesn't stint on how circumscribed and solitary her life has become -- same old house, same old (very old) neighbor, same old local dive. They're all familiar pleasures, but perhaps Kinsey needs to shake things up a little; ''Ricochet'' is so laid-back that, at its climactic moment, Kinsey takes a nap. Let's hope that S stands for Surprise; she could use one.