The media celebrities! The heartbreak! The strappy sandals! This bumptious collection of Candace Bushnell's Sex and the City columns from The New York Observer provides a prime banquette seat to witness the intense and rather frightening mating rituals of the attractive, successful, over-35-and-still-unmarried set.          Those who follow Bushnell's column will be familiar with much of the material here; indeed, a fair portion of the chapters have run in The Observer in the last six months. Placed between hard covers, however, this so-called sex column takes on a different tone — it becomes a kind of serial novel that works as both a comedy of manners and a class study of the current Age of Non-Innocence.          In her search for love amidst an endless stream of lunches and cocktail parties, Bushnell paints a bleak but funny portrait of her sisters in heels as they get everything they want except for a husband and children. We follow the intrepid, hungover "reporter" from a swingers' club (where the hottest thing was the buffet table) to a male forum on threesomes; from dinner with men who bed models to a bawdy ladies' tea where a serial dater is dissected. During the last third of the book, the voice shifts from the first person to that of Carrie (aka Bushnell). As she chronicles her relationship with Mr. Big (aka cigar-chomping Vogue publisher Ron Galotti), you may begin to understand why these womens' relationships fail.          One compelling aspect of these juicy, fast-reading pieces is that they offer an insider's view of a very elite Manhattan. Sure, names have been changed and events modified (and who knows how she records those quotes), but if you're a bold-faced-name junkie, you know who she's talking about, or can at least enjoy speculating. Bushnell delivers the bad news about love in Man-hattan in an engaging "he said/she said" style ("He gave her more drugs and she gave him a blow job"), as though she were hoarsely whispering in your ear during lunch at the Royalton.          As compelling as Bushnell can be, by the midway point of Sex and the City, the book's message is painfully clear: In her New York, locating and securing a powerful husband is, sadly, a woman's ultimate accomplishment.