Sixteen Hundred Pennsylvania Avenue has been UPI correspondent Helen Thomas' beat for fifteen years through four administrations. She must have flipped through her file of old carbons for this scrapbook of press dippings: John-John's birth, the Cuban crisis, the assassination, a Lincolnesque funeral, an ongoing dispiriting war, the ""credibility gap,"" a gall bladder operation, plenty of cowboy-style barbecues, four weddings, a huge political scandal, detente, two resignations, Kissinger, an ""instant president,"" two breast-cancer operations. . . remember when? In the beginning, Jackie comes off as a spoiled brat; JFK is warm and witty. The Kennedy era had a snobbish ""queen,"" but LBJ--bigger than life, earthy as Texas paydirt--was utterly outspoken about his power, exclaiming ""I'm the king,"" in a slightly sodden, off-the-record moment. Not even Helen Thomas--who bends over backwards to love every First Family--can defend Nixon; but her heart goes out to Pat, forced at the last to hide her tears behind dark glasses. In case you ever wondered how those asinine gifts got so much play in the press, Thomas explains why she interviewed Luci, Lynda, Tricia and Julie so often: ""good copy"" is the name of her game--and a callow daughter is much easier to catch off guard than a clam like Ziegler. Helen Thomas pries into private lives with a vengeance. She has a nose for human interest and is not above the odd hunch: that was Helen Thomas at the other end of Martha Mitchell's wire. No one will be offended by her gloss of the shifts in presidential lifestyle, fashion and decor--does an ambitious lady journalist bite the hands that feed her?--but there's a good deal of readable personal gossip amid the flattery and the ballyhoo. Since everyone from the major-domo to the kennel-keeper has turned out a crowd for the White House tour, this is a proven commodity--with a little more style and savvy.