HOLLYWOOD REQUIEM by Peter Freeborn (Simon & Schuster: $19.95; 251 pp.). Other than the numbers of zeros after the dollar signs and the style of clothing, this novel, written under a pseudonym, sounds as though it's right out of the 1940s. You half-expect Humphrey Bogart or possibly even Greta Garbo to show up. It's the story of a Hollywood ghostwriter, Dan, and his passionate affair with the screen goddess of all time. She's 54, but still incredibly beautiful and lustful; he's 41, drinks a lot (gin, mostly), but maintains the virility of a 19-year-old. Greed, jealousy and meanness weave their way through the various boudoirs. Hard-bitten though he may seem, the ghostwriter is an old softy at heart, and people use him mercilessly. As his life slides out of control, he becomes a rather sympathetic character.