I'm presently on a long overdue Gaston Leroux kick, acquiring and reading as many of his novels as appeared in English translation as I can find. THE PHANTOM OF THE OPERA is wonderful in ways that have yet to be filmed, but there is much more to him and his contribution to the horror genre than just that. In fact, in an impressive two-part article published in the final two issues of MIDI-MINUIT FANTASTIQUE (issues 23 and 24, 1970), none other than Jean Rollin wrote in depth about horror cinema's immense debt to Leroux's literary imagination -- a debt that, among English speaking horror fans, remains largely unpaid.
All films about intelligent apes who learn to kill can be traced back to his novel BALAOO (even the 1932 MURDERS IN THE RUE MORGUE contains ideas that didn't originate with Poe), Roger Corman's THE HAUNTED PALACE is based on H.P. Lovecraft's "The Case of Charles Dexter Ward" but that story was itself indebted to Leroux's darkly comic THE MAN WITH THE BLACK FEATHER (also translated as THE DOUBLE LIFE), and of course every unmasking scene from THE FLY to PSYCHO to MYSTERY OF THE WAX MUSEUM owes something to PHANTOM, and so on.
Speaking of MYSTERY OF THE WAX MUSEUM, I've often wondered about the real literary basis, if any, for Sergio Stivaletti's film M.D.C. MASCHERA DI CERA (THE WAX MASK, 1997), which articles sometimes link to a Gaston Leroux novel called "THE WAX MUSEUM" or "MYSTERY OF THE WAX MUSEUM" depending on your reporter... but no such novel exists, and no screen credit was ultimately given. In the course of my collecting, I've now discovered that a short story exists, entitled "The Waxwork Museum," which was collected in translation in a 1980 story compendium called THE GASTON LEROUX BEDSIDE COMPANION, edited by Peter Haining. A foreword acknowledges that the translation by Alexander Peters first appeared in FANTASY BOOK in 1969, but no original French publication date is given. It very likely appeared in the 1910s-1920s, possibly prior to Paul Leni's classic silent picture WAXWORKS (1924).
Of related interest: in Leroux's THE MAN WITH THE BLACK FEATHER (aka THE DOUBLE LIFE OF THEOPHRASTE LONGUET), there is a chapter bearing the title "The Wax Mask"... but it has nothing to do with the story of the film.