This is a novel with an insane protagonist. And that's a problem. Aside from mere clinical description, the most rewarding function of madness in fiction is to reveal, encapsulate or illuminate - even in extreme or grotesque exaggerations - some element of our sane quotidian lives, to make us see more clearly one of the common maladies of human nature.Some pretty bizarre behavior can qualify in that category: King Lear on the heath, Raskolnikov skulking around corners, Esther Greenwood going bats in The Bell Jar. But when it does not illuminate, when it is only grotesque, madness sinks to mere misanthropy, as in Jerzy Kosinsky's Cockpit, Suicide Note lies somewhere between.

Davis's novel is driven by the increasing madness of Tom Hazard, a magazine journalist and interviewer who is in Mexico on a story when he gets word of his father's suicide attempt. It runs in the family: his sister has killed herself some 18 months beofore. (Moreover, the book is set in 1968, a bloody and murderous year.) Hazard's mental health has never been good, we discover. He has lived vicariously - through interviews - since childhood and there is left at the core of his self. What little there is comes apart in this brief book. Deranged and morbid, Hazard picks up a Mexican chambermaid and the two of them set out aimlessly driving into the American Southwest, telling each other an elaborate and squalid repertoire of lies as they go. The metaphor here is The Arabian Nights : people must create desperately inventive fictions to fend off or conceal approaching death. At the end of the book, they come to the end of their stories and are thrown back on truth - which is admittedly boring, painful and depressing.

The structure of the book is alarmingly similar to Thomas McGuane's 92 in the Shade, in which another disturbed protagoinst (may we hazard the observation that his name is Chance?) gets caught in another character-is-destiny trap, and beguiles the time with a lady until his final - and largely self-willed - destruction. Unhappy, Hazard does not generate the same sympathy or warmth that McGuane's protagonist did. Although the metaphor is richer and more engaging Davis's narrative - read on a literal level - is ungainly and disjointed, raising far more questions than it answer, despite the obvious craftsmanship of the writing. (Harper & Row. $7.95)