Sexual Secrets of the Canadians could have been the title of this collection of absorbing, beautifully written stories by Alice Munro. It would have the advantage of stunning all those Americans who believe that Canadians don't have sex. And it would suggest the clandestine, oblique eroticism of these stories, which are filled with lustful intrigues, secret desires, secret histories. Several of them concern adultery in small Ontario towns, where lovers meet with the furtive precision of spies exchanging microfilm and are sometimes found out by meticulous counterspies to whom they are married. One of the most striking and darkly comic scenes in the book, Friend of My Youth occurs in ''Oranges and Apples,'' when a man returns home in mid-afternoon and doesn't surprise his wife in bed with her lover. Instead he sees her sunbathing in the yard and senses from her slightly self-conscious, enticing movements that she isn't really alone. Picking up binoculars, he spots a friend sitting at the window of an apartment overlooking the yard. He is apparently naked and is watching the wife through his own binoculars. The husband completes the tense, voyeuristic triangle: ''And looking at Barbara he could feel the glow along the surface of her body, the energy all collected at the skin, as she gave herself up to this assault. She lay not quite still there was a constant ripple passing over her, with little turns and twitches. Promising no, she was already providing the most exquisite cooperation. It was obscene and enthralling. Unbearable.''
The mystery of people's hidden lives, of which sex is just one particularly mysterious aspect, is the most persistent theme of these stories, and the partial revelations they offer give them their suspense. One story, about a Canadian woman who visits Scotland, where her late husband had spent time during the war 40 years earlier, takes on a Dickensian atmosphere of grotesque comedy and reclusive mystery as she meets baffling women out of his past. Like most of the stories in this volume, it has the chronological span and rich texture of much longer fiction. Nietzsche once wrote that it was his ambition to say in a sentence what other writers say in a book-or rather what they don't say in a book. Alice Munro tells us in a single story what other writers tell us or fail to tell us in an entire novel. A