THE ANDY WARHOL DIARIES edited by Pat Hackett (Warner Books: $19.95, illustrated). Pat Hackett, the artist's de facto secretary, culled these excerpts from the 20,000-page original manuscript, which must have been a daunting task. The notorious "Diaries" offer little insight into Warhol's creative process: He rarely wrote about what he working on, although he was careful to note the prices earlier works were bringing at auctions. Instead, he focused on the most mundane details of daily life, and it's amazing that an artist who created such influential work could be so preoccupied with such banal trivia. Although he was extremely rich, Warhol fretted about the cost of cab fares and movie tickets, and how many copies of Interview he handed out in a given day. The name-dropping accounts of parties, discos, celebrities, hangers-on, drugs and booze--which put "Diaries" on the best-seller lists in 1989--prove just how uninteresting decadence really is up close. Must reading for Vanity Fair subscribers who want to know what Halston said to Liza and Liz and Bianca and Mick and Marisa . . .