THE RUBY SLIPPERS OF OZ <i> by Rhys Thomas (Tale Weaver Publishing, 636 N. Robertson Blvd., West Hollywood 90069: $14.95; 232 pp.) </i>

For collectors of film memorabilia, there are a few items that embody all the magic of movie lore: Sam’s piano from “Casablanca,” the Rosebud sled from “Citizen Kane,” Charlie Chaplin’s bowler hat and cane. Rhys Thomas discovers that the ruby slippers worn by Judy Garland in “The Wizard of Oz” are perhaps the greatest obsession of film collectors. They are, in the words of Jack Haley, son of the actor who played the Tin Woodman in the 1939 MGM film, “Hollywood’s equivalent of the Holy Grail.”

“The Ruby Slippers of Oz” tells a sort of detective story. In the course of research for a television spot, Thomas accidentally discovered that in the original “Oz” screenplay--as in L. Frank Baum’s book--the slippers were silver. Silver just wouldn’t do, however, for a film being shot in expensive Technicolor. The slippers whose magic would transport Dorothy home to Kansas became ruby-colored, glittering with hundreds of sewn-on sequins.

Thomas also discovered that the pair of ruby slippers sold for $15,000 in 1970 were not the only pair in existence. In fact, as many as seven pairs might have been used in the production of the film; Thomas set out to discover what became of them. The research plunged him into the bizarre world of collectors, beginning with the 1970 auction of the contents of four studio lots at MGM, where items to be disposed of included chariots from “Ben Hur” and a full-sized ship from “Mutiny on the Bounty.” A young costumer, Kent Warner, was hired to sort and arrange costumes for the sale. Warner is supposed to have discovered a cache of ruby slippers and to have kept several pairs while leaving the worst pair for the auction.

The trail of the ruby slippers turns up great stories, as well as much lying and cheating on the part of people who want to acquire the shoes--or people who want to believe that their pair is the one true pair worn by Judy Garland. Thomas concludes that the absolute facts about the ruby slippers will remain forever murky. In several years of research, he writes: “I had not learned so much about those red sequined shoes as I had about people.” This week marks the 50th anniversary of “The Wizard of Oz,” one of the silver screen’s most long-lived obsessions.