PEOPLE READ thrillers for fun and heavy books for heavy purposes. The odd thing is that, at their best, the thrillers of the past generation have provided the clearer and sharper descriptions of the world in which we have been living. Future readers who want to recapture the atmosphere of international Europe at certain moments in the recent past will have no sources better than the novels of espionage by Eric Ambler and John LeCarr,e. Similarly, for people who want to know about 20th-century California the central author will be Kenneth Millar. He wrote two dozen de- tective stories, most of them under the name of Ross Macdonald, and he died this week in Santa Barbara.

The clich,es about California always speak of rootlessness and transience. But the Macdonald plots move back and forth across two or three generations through events that happened in the same places over much time. There was a lot of autobiography in his work; his father had left his mother shortly after he was born, and the search for the lost father or the lost child turns up repeatedly in the books.

Murder never interested Mr. Millar much, for a man who made his reputation by it. He used it only as a device to flush people out of their protective habitats. The point of the stories wasn't death but the consequences of death revealing history and intention. The crime that chiefly attracted his interest was betrayal of trust--between husbands and wives, between children and parents, sometimes of patients by doctors. It was all set in a California landscape evoked with accuracy and force.

His books usually start off conventionally enough, with the standard puzzle of who killed Cock Robin. But shortly things get much more complicated. As a critic once observed, in these books guilt rarely lies with only one person. The affair turns out to be more complicated than that, and more interesting.

That's the great danger in this kind of good writing. You innocently begin to read, wanting nothing more than to be amused. Then, drawn along by an irresistible plot, you begin to discover more than it is comfortable to know.