A CONSTANT LOVER by Paul Rosenblatt (Watermark Press: $17.50; 235 pp.). This is a who-am-I story narrated alternately with soft-focus poignancy and half-drunken cynicism. The hero wafts back and forth between life back in the States and his travels in Italy, having amusing but inconsequential encounters, fantasizing about his marriage and his love life, and gadding about the edges of the art world with a can of spray paint. His main concern apparently is to seize the moment, to have something real happen--sex, fame, an interesting chat--to make him feel alive. But the passages that seem most vivid are those regarding food and drink. In the realm of the senses, the would-be sexual hero is more eloquent about bellying up to a table set for a feast than about his relationships with his wife and other women.