ARNOLD RIGGS "stare[s] at the figure of Buddha who said that all suffering was caused by desire." Sexual longing for unreachable partners, carried to a metaphysical extreme, drives David Helwig`s new novel, Of Desire (Viking/Penguin, 231 pages, $24-95 cloth). The plot is thin: Ross Riggs has apparently drowned himself. His second wife, Sandra, along with Arnold and Donald, his two sons by his first wife, recall the past and live through the days until Ross`s body is found and buried. Most of their thoughts and actions revolve around sex: Arnold`s juvenile lust for Sandra when she was his new stepmother; his yearning for his daughter Julia`s lesbian lover Elena; the misunderstandings and frustrations between Elena and Julia; Donald`s affair with his sluttish tenant and his neglect of and reconciliation with his wife; and so on. None of the many permutations of sexual desire is unique in itself, but together they add up to a provocative exploration of this universal human feeling. In fiction, sex always used to be seen from a heroic (or anti-heroic) male perspective, and female sexuality was simply male sexuality in reverse, whether the author was male or female. In Of Desire Helwig treats male sexuality honestly -- Arnold and Donald display a range of emotions about their own sexuality that I have not seen before in fiction -- and his depiction of female characters shows that he has paid attention to what women have said about their experiences. Sexual desire aggravates loneliness because, by projecting the fantasies of the desirer onto the desired one, it prevents understanding and intimacy. Of Desire is a masterly exploration of this sad truth.