BILLY LIAR (191 pp.} Keith Water-houseNorton ($3.95).
Billy Fisher is hardly an angry young menace, but he might be mistaken for a highly comic younger brother of Play wright John Osborne's backward-scowling Jimmy Porter. At 18 or so. he is a mortician's clerk in a scruffy little Yorkshire town, so benumbed by his surround ings that he fancies he has caught an entirely new disease, Fisher's Yawn. When his earthbound parents mulishly refuse to, understand his plans for becoming a scriptwriter in London, he retaliates in his imagination by inventing a set of properly sophisticated, London-based par ents, including a frightfully U...