Just when did Robert B. Parker, the most gifted hard-boiled mystery writer of the 1970s, start down the road to self-parody? A lot of fans would point back about 10 years or so to the seventh Spenser book, Early Autumn. There, Boston's toughest detective played surrogate dad to a troubled teenager named Paul Giacomin, giving the kid lessons in manly independence. In fact, Spenser spent so much of Early Autumn lecturing Paul on ''autonomy,'' or preening over his own virile self-sufficiency, that he didn't do much detecting. And from that point on the annual Spenser adventures seemed to get thinner and thinner, with empty action replacing mystery and one repetitious theme a smug Spenser rescues some pathetic weakling substituting for genuine character interest.
So one doesn't approach Pastime, billed as a sequel to Early Autumn, with the highest hopes. Just as well; unlike last season's better-than-average Stardust, this predictable melodrama is par for the recent Parker course. Paul Giacomin, now 25 and a performance artist down in New York City, asks Spenser to help in the search for Paul's mother, Patty, who has vanished from her home in Lexington, Mass. Soon it becomes clear that Patty, always a pushover for a third-rate guy, has taken off with her latest boyfriend, creepy Rich Beaumont, a bagman for Boston gangster Gerry Broz. What Patty doesn't know, however, is that Beaumont has stolen a fortune in cash from the mob; the mob wants it back, naturally, and wants Beaumont dead, which means that, to save Patty, Spenser will have to outshoot and outsmart a whole posse of vicious gangsters. No problem.
Parker fills out this ho-hum scenario with his usual slick mix of laid-back comedy, macho sentimentality, and matter-of-fact violence. There are also, thrown in here and there like Hamburger Helper, Spenser's recollections of some highlights from his youth which presumably help to explain how he grew up to be Mr. Autonomy. ''You are the most self-sufficient man I have ever known,'' gushes Spenser's girlfriend, Susan Silverman. ''You're probably peerless, there's a kind of purity you maintain.''
For those who worship Spenser as much as Susan does, Pastime will be a particular treat. Less adoring readers are likely to be painlessly, if forgettably, entertained. But it's hard not to be distracted by reminders of Parker's early promise, by hints of unfulfilled potential. (By far the most interesting situation in Pastime the relationship between inept Gerry Broz and his despairing mob-boss father is only barely sketched in.) The more you know about Parker and Spenser before Early Autumn came along, the less you'll be satisfied by Pastime. B-