Robert B. Parker is a tough act to follow - especially for Robert B. Parker. now that he's written more than 20 crime novels that pretty much set the standard for the private eye mystery, what's left? How far can he take the Boston-based gumshoe he calls Spenser, and will this muscle-bound man of honor ever show his age? For all his periodic declarations of feeling past his prime, Spenser has stayed an eternally young and virile strongman who sticks to his principles and shrugs off the body blows of time - until now. In Small Vices Mr. Parker not only brings his hero to the point of death by challenges him to confront his own mortality in a way that he hasn't since "Valediction" (1984). Surprising for a book that is something of a milestone, the frame of the story is one of Mr. Parker's more insubstantial efforts. A young black man, falsely imprisoned four years earlier for the murder of his white college sweetheart, needs an honorable defender to clear his name and spring him from the slammer. Spenser is just the guy to do it and he almost does, until he is shot by ''the Gray Man,'' a mysterious hit man who looks like some spooky medieval icon for Death. (''His face was without expression. His bearing without affect.'') Spenser and his nemesis meet again, but not before an extraordinary 10-month hiatus, during which time the paralyzed detective (who sardonically refers to himself as Lazarus) hides out in California with his soul mate, Susan, and his friend Hawk, fighting his way back to life.

Mr. Parker has written a powerful piece about the defeat and reclamation of a hero, but I wouldn't say that Spenser's dance with death teaches the old knight to act his age. On the contrary, his morbid experience convinces Spenser that he must defy his mortality - not by exercising the conventional options of having a child or transferring more functions to his mind, but by rebuilding his damaged body parts. ''If I'm to pursue my chosen profession,'' he reasons, ''I can't be . . . slow and clumsy.'' And there's no chance of his giving up the profession. ''It's the only thing I'm any good at,'' he says.

Although Mr. Parker once criticized Rex Stout for allowing Archie Goodwin to remain a frisky 32-year-old throughout his 40-year career, Spenser goes Archie one better in ''Small Vices.'' By virtue of his mythic death and rebirth, he has defied mortality altogether and become like some fertility god who lowers himself into the ground each winter and comes roaring back to life each spring. I say good luck to him.