THE NANTUCKET DIARY OF NED ROREM: 1973-1985. By Ned Rorem. (North Point, $30.) Frank O'Hara is said to have remarked that Ned Rorem's ''Paris Diary'' ''reads like 'The Alice B. Toklas Cook Book' without the recipes.'' In his ''Nantucket Diary'' Mr. Rorem corrects this omission. Kept between the ages of 49 and 62, a period during which Mr. Rorem took up part-time residence on Nantucket, ''The Nantucket Diary'' is where the Pulitzer Prize-winning composer does his personal, professional and household accounting. Mr. Rorem catalogues meals consumed, physicians consulted, guests entertained, music commissioned, reviews written, books read, grudges nursed, memories savored. For all the proper names, musical, famous and otherwise, that appear in these pages, the book does not trade in gossip; there are but one or two anecdotes one itches to repeat. This volume, less sexually and emotionally charged than earlier installments of his ''Diary,'' chronicles the postdrinking, postsmoking, and seemingly postprofligate years; by his own admission, Mr. Rorem has come to value discretion. ''The mood is quieter. After a certain age, certain subjects become embarrassingly dull and nobody's business. One of these is sexual intercourse, the other is the injustice of personal sorrow.'' The intimacies recounted here are those of daily life, and, not unlike daily life, the entries often repeat themselves. The author's subjects are, for the most part, disarmingly benign: work, his sweet tooth, friends and his elderly parents, about whom some of the most affecting passages in the book are written. Mr. Rorem's candor and wit help relieve the self-consciousness and stiltedness that diaries written to be read are susceptible to. ''The Nantucket Diary'' is engaging, not profound. For this Mr. Rorem makes no apology: ''As though the very nature of diaries were not based on the Importance of Unimportance.''