BOOK REVIEW : A Murky Mystery on Cape Cod : AT SEA <i> by Toby Olson</i> ; Simon & Schuster $19, 208 pages

An unpublished draft of the jacket copy for this novel claims that its prose is “totally excessible.” But be warned: If it really was “totally excessible,” the people trying to sell it to you wouldn’t have to mention it.

This is the kind of story you walk around the house with, reading scraps out loud, not because you think they’re beautiful, but because reading them out loud seems to be the only way to get a handle on what they mean: “It isn’t revenge now,” a gay milkman-piano player remarks on Page 131, “it’s simply the correct thing, and though I know it’s for me, it’s to get right in the mind, out in the world, not in body. Not that any more. She’s really gone now, and mostly I can remember her clearly.” “Totally excessible,” for sure.

I tried to go with the flow so that when a nice barfly describes chasing a criminal, I didn’t even get upset, just bemused: “Stub was spinning, off to the side, and my boy Allen here, having tripped at stool--that small circle of bar support near bottom--was fallen almost headwise, and a good thing Sara is fleet of limb and caught him before he saw stars and lost Clifford, the last note in the middle of which was still a drown.”

Is this what they’re calling English nowadays? Why sure. And it’s totally excessible.

If you were to scrub down this prose with Formula 409, the pattern underneath might look interesting and even attractive. Peter Blue is a 42-year-old police detective on Cape Cod. He’s a womanizer, compulsively cheating on his nice wife, Sara. He’s told her about this cheating, for a reason never revealed.

Peter must keep order in a previously closed and stable society that has become increasingly complex. Not only are there cultural differences between summer people and all-year people, but perhaps half the population of the Cape is gay. Not only that, the drug-running business is the community’s growth industry. Hard to figure out when you’re an old-fashioned heterosexual guy who drinks too much and cheats on his wife, but Peter tries.

Then a girl gets raped. A couple of weeks later she’s attacked again and then murdered. Peter has already been strangely attracted to her, but then he’s strangely attracted to almost every female.

The novel breaks in the middle, when Peter becomes friends with the dead girl’s father, Charlie, the milkman-piano player previously mentioned. Suddenly, the whole world shifts: Peter can see things he hasn’t seen earlier. His wife leaves him and comes back as his friend; Peter begins to hang out in a very welcoming gay bar--called, with heavy significance, the Blue Boy--where Charlie plays.

People start having a nice time, going out fishing in the bay, eating up big lobster dinners, listening to Charlie more and more, so that the lyrics of the great old jazz standards become part of the narrative. And no matter how murky the prose, a novel where characters discuss the relative merits of Thelonius Monk, Chet Baker and Lennie Tristano can’t be all bad.

But the author has written 15 previous volumes of poetry, and he seems to have inherited the contempt that many 20th-Century poets have for the middlebrow audience. He evidently feels that he doesn’t have to explain anything to his readers. He’s writing for the happy few.

Except mystery stories with murders and drug busts ought to be for more than the happy few. I hate to break the news to the author, but characters in a mystery novel ought to talk like human beings, not like the Delphic Oracle. The “mystery” should lie in “who done it,” not in “what in the world are they talking about?” A mystery really should be, just a little bit . . . accessible.