I became an admirer of Leonard Michael's fiction when I read The Men's Club twenty years ago. The Men's Club is a short, mordant novel about seven men who gather in the Berkeley hills to discuss life, hardly a topic I can imagine tackling with any six of my male friends (for me it's a one-on-one subject; even if there were only three of us, one would be left out.) In the end, they talk more about the women in their lives than anything else, revealing the pains and resentments and confusions men harbor toward half of humanity while clarifying little and drinking too much. Sound boring? Well, it's only 132 pages, and Michaels wrote with great economy and deadpan vivacity, capturing a character's saga in very few words.   As the years went by, I dipped into Michael's short stories with interest but not much enthusiasm until he began publishing what are known as the Nachman stories. Nachman is an academic, a mathematician, who lives in L.A., has very little social life, and yet somehow is drawn into complex ethical and moral situations that defy resolution by his rational, analytic skills. In a sense he's a classic straight man, a sort of second banana, a recipient of referred pain and slapstick knocks on the head from history, culture, and his best friend's wife. The fundamental joke is that everything makes sense when Nachman sits alone working on math problems--and nothing makes sense when anyone intrudes on him.   Michaels was often praised for his short fiction, particularly his devastatingly concise style, but the Collected Stories of Leonard Michaels really reads like a prelude to the Nachman stories, seven in total, that appear at the end. They are the summa, his major accomplishment, perhaps because in Nachman he developed a character who is not transient, who absorbs punishment without being extinguished by the pain, comic or otherwise.   I wouldn't recommend skipping his earlier stories to get to Nachman, but I'd suggest dipping into them selectively and then leaping ahead to the end. It would be a pity to grow tired of Michaels' droll, comic send-ups before settling in with Nachman and reading Michaels at his best.