Seven springs ago, Alec Wilkinson, 23 years old and a recent college graduate with a major in music, was hired as a policeman by the small Cape Cod town of Wellfleet. He became the ninth member of the force, and resolved to stick it out for a year on the grounds that the experience "would do me good." That he had absolutely no qualifications for the work seems to have made little difference to the chief who hired him; he managed, in any event, to get through the year without doing significant harm to himself or anyone else, and to emerge from it with this agreeable, mildly amusing book.

As the least experienced member of the squad, Wilkinson drew the least desirable shift: 8 p.m. to 4 a.m. His principal enemies were boredom and somnolence, his chief pleasures the midnight secrets of the town that he came to know during the months he patrolled it on foot and by auto -- what another policeman described as "snooping around, being on the prowl." His closest brush with serious crime involved a boat whose crew was attempting to make a large nocturnal delivery of marijuana; the main effect of the experience was to bring home to him that "you're not quite as safe as you think you are here, with nothing to do at two in the morning except chase dogs."

Much of his time was spent in idle conversation with the other members of the police force. Over the months he developed a strong feeling for them:

"I admired all the men I worked with. Nothing about any of their lives was easy, they worked terrible hours and were deprived of their families . . . they made no money to speak of, no one outside their small group liked them or felt any sympathy for them, and everyone wished them troubles, but I never heard them complain. They were generous and patient and always extremely kind to me. Again and again I gave them reason to throw up their hands, but they never did."

Wilkinson's year on the force, though a long one, had among its rewards the custody of the guns and cruisers that come with the policeman's job. He never became especially adroit at driving the big cars, and to his relief he never had to fire his gun in the line of duty, but he got a considerable and entirely understandable post-adolescent kick out of having them at his fingertips. Still, the pleasures of the job were outweighed by its shortcomings:

"I was glad when the year was up. I felt I'd survived something. I never cared for the things I had to concern myself with -- warnings, tickets, arrests, confrontations, problems. For the most part they were small-minded. It was depressing to be disliked by people I didn't even know, depressing to be cursed in public and to work midnight-to-eights over and over, depressing to harass the kids on Main Street only because the selectmen wanted it done."

In making that observation, Wilkinson indirectly and inadvertently calls the reader's attention to his book's primary shortcoming. Unlike other members of the force, Wilkinson did not face a lifetime of the "depressing" side of the policeman's lot. Wilkinson, the son of New Yorkers who owned a summer house in Wellfleet, was always free to pack it in and get on with what one assumes is the considerably more glamorous, less depressing life he now leads as a staff writer for The New Yorker. Although Wilkinson seems to be a level-headed and self-effacing fellow, there is nonetheless about his year as a cop an air of slumming, of a brief lark among the common folk.

Similarly, Wilkinson's narrative conveys a pervasive sense that he is not being entirely forthcoming with the reader. Was the real purpose of his year on the force to do himself "good" -- or to write this book? If he did not have a book in mind when he began, how can one explain the long quotations from other members of the police force, words that were uttered seven years ago in distinct, discreet voices? The reader must be pardoned if he suspects almost from the outset that "Midnights" has less to do with private experience than with "personal journalism."

It's too bad that the book raises these suspicions and objections, for as a portrait of the policeman's life in a small town it is very effective and probably quite accurate. It is also pleasant, diverting reading.