A superhero comic book without a superhero, Neil Gaiman's The Sandman is a most unlikely hit. Six years ago, this English writer took a defunct American comics character and turned him into a figure of voluptuous gloom and doom. The Sandman enters dreams, helping good people, creating nightmares for bad people, and generally messing with everyone's mind. Gaiman, 33, says, ''Superhero comics are the most perfectly evolved art form for preadolescent male power fantasies, and I don't see that as a bad thing. I want to reach other sorts of people, too. I'm proud that The Sandman has more of a female readership, and an older readership, than DC Comics has ever had.'' A Sandman poster has been seen on Sara Gilbert's bedroom door on Roseanne, Seinfeld's Jason Alexander wore a Sandman T-shirt in Esquire, and singer Tori Amos has sung about him. Currently, The Sandman is one of DC's best-selling titles, after Superman and Batman. ''It's an ongoing novel stuffed with myth, history, and literature,'' says Gaiman. ''I'm writing the kind of comic I'd like to read.''