BIRTHDAY PARTIES are fun, even though we've grown too old to take jelly beans home in paper baskets. Bill Targ had his 70th birthday recently and celebrated his three-score-years-and-ten with a huge party and a new two-year contract with Putnam, where he's a senior editor. Looking tanned and terrific, Targ pushed his way through the African art that adorns his penthouse and the crowd of big-name admires to tell his columnist of his irrespressible enthusiasm for the parts of the new Mario Puzo novel that he has seen . . . Peter Rabbit had his 75th birthday, and Frederick Warne & Co. threw him a huge party at Windows on the World Restaurant, where they introduced Peter Rabbit's Natural Foods Cookbook . . . Holt, Rinehart & Winston held Erica Jong's birthday party at Brentano's Bookstore (she's about half of Bill Targ's age) and hundreds of people milled around the first floor of the giant emporium, munching canapes and looking for celebrites . . . Judy Lynn and Lester Del Rey were given a kind of "Birth Day" party to celebrate the birth of their brand new science-fiction imprint under Ballantine's aegis, Del Rey Books. The flowers were from Leonard Nimoy, the guests were from the ranks of the science-fiction writers and the weather abominable. It was yet another party atop the World Trade Center at Windows on the World, and the windows were all fogged up. You couldn't see a thing, and Ron Busch was threatening to demand a raincheck from the restaurant.