"My name is Lily Campendonc. A long time ago I used to live in Lisbon. . . ." So begins Cork, in which a pragmatic young British widow, proprietor of a cork-processing factory, embarks on an affair with her husband's former office manager, Her lover will meet only once a year, and insists that their encounters be pseudonymous masquerades. For him, this role playing is essential: ''You see, because I am nothing, I can imagine anything. . . . If I were something, I would be unable to imagine.'' Punctuating the story are excerpts from a monograph on the peculiar traits of cork trees. injecting a seemingly dry scientific treatise into a tale of passion. As it turns out, the rare characters of cork''lightness, impermeability, elasticity''and the unusual nature of the love affair have much in common.