TO A PUBLIC FAMILIAR with sagas and sequels, Avon, come next January, is introducing a "new" old idea. Dickens did it and Trollope, so why not a 20th-century writer who's as prolific as some of his Victorian forebears? Thus, Michael McDowell whose earlier books include Cold Moon Over Babylon, Gilded Needles and The Elementals, will see his next novel, Blackwater, appear in six parts, one a month for as many months. "We'll probably run an ad in the back of the first one offering the rest at a discount if they subscribe," says Bob Wyatt, Avon editorial director. And to whet appetites, he claims that in each part "we guarantee a cliffhanger, a resolution, and something truly horrendous."

Going on to describe Blackwater as a "matriarchal, multi-generational, Southern occult novel, going from World War I to the present day," Wyatt mentions that he has encountered an unexpected difficulty while editing this pet project. Since the manuscript is coming in pieces, with McDowell working in installments as Dickens himself did, Wyatt had planned to edit first and read later. But to his surprise, he soon found himself slowed down by the urge to put his pencil aside and simply read.

Before beginning Blackwater McDowell had written seven novels for Avon--that's since 1980--two with a collaborator under two different pseudonyms. For Vermilion, a mystery featuring a gay detective, he and Dennis Schuetz called themselves Nathan Aldyne. For Blood Rubies, a gory gothic tale of twins separated at birth, they chose Axel Young. Then earlier this year St. Martin's published in hardcover a sequel to Vermilion, called Cobalt; Avon will do that in paper this winter. (A third in the Aldyne series, keeping to the John D. MacDonaldish color theme, is called Slate and is due out from Avon in '83.) Says Wyatt with a note of disbelief in his voice: "We've got eight books altogether of Michael's scheduled for next year--the six parts of Blackwater, the Aldyne and a new Axel Young, which is called Wicked Stepmother." Of course, there's also Katie, a McDowell solo title coming from Avon this fall.

Charles Dickens died before he could finish the novel he was working on in 1870, The Mystery of Edwin Drood. Only half of the book's 12 parts were finished; the intended ending remains forever an enigma. Naturally not wishing any harm to McDowell, still one couldn't help wondering about his plans for bringing the curtain down on the events of Blackwater. (Bob Wyatt has so far received only five of the sections.) Unperturbed at being asked, McDowell in fact pronounced the question timely. "I just figured the end out yesterday." Has he put it on paper? Well, no, or only a page of notes. But those, he warns, "won't be of any help to anybody. It's mostly still in my head." We can only hope that next June will indeed bring Rain. . . the title of Blackwater's concluding volume.