Charles Dickens planned to produce ''The Mystery of Edwin Drood'' in 12 monthly parts. The first installment appeared in his weekly All the Year Round in April 1870. When he died very suddenly of a stroke on June 8th of the same year, only six numbers were completed.

The setting was for him a new and narrow one — the society of the cathedral close of a provincial town in the county of Kent. Dickens, with ill health and depleted energies, spent a great part of his time in a country house he had acquired in that very county. Nevertheless, the novel opens in an opium den, and, indeed, in the opium dreams of an educated man in a lowdown district of east London. We soon learn, however, that this same man is Jasper, the courtly, talented, wellbred choirmaster of the cathedral. The same man, yes, but apparently without conscious awareness of his other life. This is the reader's first mystery. Then comes the disappearance of Jasper's ward, Edwin Drood, a young man in his early 20's, after a supper at his guardian's and a tipsy quarrel with another young man, Neville Landless, who has come from Ceylon to be tutored in Latin and Greek by the manly, sports-loving dean of the cathedral, Crisparkle. But there is no trace of Edwin's body. Is it murder? Is the obvious man, Landless, the villain? Or is it the seemingly respectable, opiumtaking Jasper, whom we soon learn is passionately in love with his ward's betrothed?

It is hardly surprising that critics ever since have occupied themselves with suggesting — or ''proving,'' as so many of them thought — what had been the author's intended ending. A bibliography by B.W. Matz published in The Dickensian in 1911 notes 82 such attempts, and Matz's daughter Winifred added a further 135 by 1929. In 1930 Howard Duffield offered the principal American solution, that Jasper was supposed to be a member of the Hindu sect known as the Thugs, who were devotees of Kali, the goddess of destruction. This theory was to play a large part in Edmund Wilson's famous essay on Dickens in ''The Wound and the Bow'' (1941). In 1964 the well-known English actor and devout Dickensian, Felix Aylmer, offered the most ingenious and carefully worked approach of all, as Charles Forsyte says in ''The Decoding of Edwin Drood,'' an Agatha Christie solution which turned Dickens's book, as we now have it, upside down. Now we have suggested endings by two masters of mystery fiction offered us in the same year: Leon Garfield's invention of the final chapters as Dickens might have written them (to be published on March 15) and Charles Forsyte's conclusion, drafted in a contemporary idiom. Such is progress in the popular arts, it is clear at once that these are (though different) the two most convincing and entertaining specimens of all the Droodiana, as these speculations have come inelegantly to be called.

Here, however, I should declare my bias. For me Charles Dickens is one of the five or so greatest novelists of all time — a unique mixture of conservatism and innovation in his craft, of imaginative genius and muddled thinking in his view of life, the novelist whose fiction subsumed both drama and popular theater in away that hasn't been sufficiently exploited since. On the whole, however, I must say that his Achilles' heel tends to be the ending of his books. The weakest aspect of the endings of many of his novels is the belief that happy matrimony can be tacked on to the most complex and original stories for the edification of his simplest readers. This in itself is connected with his primary weakness, his insistence upon inventing pretty-pretty cutouts rather than living heroines. But it must be said that, roughly from ''Bleak House'' onward, Dickens's heroines grow steadily more complex, less angelic and stronger. The two Drood heroines, Rosa Bud and Helena Landless, are perhaps the most real of all his women.

Another, more dramatic sort of ending is the way the powers of evil are destroyed in the final pages of his books; but suspenseinducing though the fates of his adulterers and murderers are, there can be little doubt as to the outcome. It is the complex nature of Dickens's evil men, not their merited fate, that makes them the peers of Dostoyevsky's lost souls. For this reason, I have always been irked by the critical treatment of his last novel as a pure whodunit. ''Endings'' were not his strong suit.

Let me say that both of the books under review have the good sense to accept the clues offered us by Dickens's friends and illustrators rather than subordinating such evidence to their own fancy, as most solvers have done; both take full account of the title-page illustration by Charles Collins and the emendations of it by Luke Fildes, and the many clues to the novelist's ultimate intention that it provides (the issue being whether Jasper's long black scarf is the instrument of Drood's murder, and whether Dickens confided this detail to the illustrators). Nevertheless, both Mr. Forsyte and Mr. Garfield are more determined to delight us by bringing Dickens's novel alive than to remind us that the half of the novel we have offers no serious clue to the half that was to come. Insofar as they accept Jasper as the murderer of Drood, they are both set in a sensible direction. Insofar as they are concerned with Jasper as a divided personality, I am sure that they are right, although I think that concern with earlier aspects of Dickensian psychology — the triple personality of Flora Finching and Mr. F.'s aunt in ''Little Dorrit,'' Mr. Pecksniff warming his hands benevolently as though they were another's, Mrs. Gamp's invention of Mrs. Harris -would have been much better clues to Dickens's belief in the divided self than external clues like Dickens's interest in juggling or mesmerism.

Mr. Forsyte's principal difference from Mr. Garfield — his belief that taking opium is Jasper's escape from his psychologically divided personality rather than its cause — is probably right. But I am not sure that it helps the novel's development. One of Mr. Forsyte's most interesting inferences from this is his decision to end the book by having Jasper commit suicide in the prison cell when he cannot procure opium to escape his murderer self; and Mr. Forsyte justifies this by a brilliant piece of word analysis of a portion of the text as we have it. Yet Dickens's obsession with public hanging, the other side of the coin to his obsession with murder, is so pronounced that Mr. Garfield's public execution scene results in a far finer climax than Mr. Forsyte's suicide scene.

In short, I must declare that Mr. Garfield's audacity in presenting his version of the last half of the novel in a pastiche of Dickens, following on directly from the authentic text of the first half, pays off wonderfully. The language is nearly always believable as being that of Dickens himself. On occasion the contents go wrong, as when the young ladies at Miss Twinkleton's Academy, recalling their summer vacation abroad, think of ''the dark-eyed waiters'' they have met: Dickens might have allowed a degree of juvenile amorousness to young ladies in his later novels, but that one of their memories should have been of ''dark-eyed waiters'' is surely impermissible, not so much on sexual grounds as on those of class distinction.

But excellent though Mr. Edward Blishen's short introductory history to Mr. Garfield's text is, it is not a substitute for Mr. Forsyte's brilliant analysis of the Drood problems. On the other hand, I do object to Mr. Forsyte's suggestion that Dickens was ''engaged in writing the first great psychological crime story in our literature'' as being unworthy of a lover of Dickens.

However, let us hope that these two excellent books will convert many mystery readers into readers of Dickens and, by their success, deter other mystery writers from trying their hand at further solutions to the mystery of Edwin Drood.

Angus Wilson