A dumpster’s worth of news about the unsightly deterioration of the British royal family has recently changed the way Di-besotted Americans like myself pore over the promotional magazine Majesty. No longer do we maintain quite the same awed appreciation of the Princess of Wales’ gracious smile or the Queen Mother’s lovely feathery hats. On the other hand, a revisionist understanding of the House of Windsor as a calcified, anachronistic institution perpetuated by flawed human beings about as regal as potatoes frees Di-besotted Americans like myself to devour three new books about the recent royal fiascoes with a hearty appetite. (A fourth and fifth, A.N. Wilson’s The Rise and Fall of the House of Windsor and The Tarnished Crown, by Anthony Holden, are both due out later this summer.) How you rate each depends on how you like your royal roast. If you favor hypocritical, self-serving cluck-clucking disguised as important intelligence about ”what went wrong in the romance that never was,” try James Whitaker’s DIANA VS. CHARLES: ROYAL BLOOD FEUD (Dutton, $22), written by a longtime royals-beat reporter now at London’s Daily Mirror. Whitaker is boastful and oily and insinuates himself into the picture at every turn. ”It was at this point that a bond developed between Lady Diana and myself that I have cherished ever since,” he swoons, moist-eyed, at one point. But devotion to the princess does not prevent him from reporting (as do the other journalists) that the uncensored ”Squidgy tape” of conversation between Diana and her Special Friend, James Gilbey, included a discussion of masturbation. Whitaker dislikes Charles and rolls his eyes at the prince’s married squeeze, Camilla Parker Bowles. His big bang: Charles and Diana’s bedroom was bugged for years, possibly by government insiders. The book includes transcripts from the taped ”Camillagate” conversation between Charles and Mrs. Parker Bowles, as well as standard color pictures. For his pink-faced exertions, give him a C, as in Charles. If it’s a slapdash, bitchy book dotted with divots of dirt you fancy, go for BEHIND PALACE DOORS: MARRIAGE AND DIVORCE IN THE HOUSE OF WINDSOR (Putnam, $22.95) by Daily Mail gossip columnist Nigel Dempster and his fellow journalist Peter Evans. The book is nominally about marital problems throughout the royal family. But for Dempster and Evans it really gets down to details like this: ”Much to Philip’s surprise-no, astonishment- Elizabeth discovered sex on her honeymoon and maintained a decidedly keen interest in it from that point on.” These authors, too, are pro-Di and anti-Charles. The big bang: Charles was ready to drop out of the prince business and move to Tuscany after Camillagate, but his grandmama convinced him to stay put. Behind Palace Doors features cheap paper, big margins, large type, and poor-quality black- and-white photos. Give it a D, as in Diana. Now, if it’s the contrarian, Loony Party view you prefer, you’ll want to go straight to THE ROYAL MARRIAGES, by Lady Colin Campbell (St. Martin’s, $19.95). Campbell, the author of Diana in Private, hates the Princess of Wales (”She will eventually become the Zsa Zsa Gabor of royalty”) and, indeed, the whole Spencer family (”they were never as great as they thought they were”). She admires Charles. But she is positively gaga over Philip, whom she finds compellingly sexy (”well known for the Schleswig-Holstein-Sonderburg- Glucksburg appeal, which coursed through his amorous veins”). In Campbell’s hyperventilating universe, dangerous liaisons are common (including, she hints, for the queen) and everything would be fine if only that awful Spencer girl hadn’t mucked it all up. The Royal Marriages skips the Squidgy transcripts; it does, however, include a nice section of color photos with nutty captions. I don’t trust this Campbell personage as far as I can throw a polo mallet, but for sheer madness, give her book a B-, as in bonkers.