Melville sees a ghost(writer)

Authorship of the Book

There seems no reason to suppose that Owen himself wrote the Narrative.  It bears obvious tokens of having been written for him; but at the same time, its whole air plainly evinces that it was carefully & conscientiously written to Owen's dictation of the facts.  ——  It is almost as good as tho' Owen wrote it himself.

-- Melville's "Memoranda in Chase's Narrative," reproduced in the 1988 Northwestern-Newberry edition of Moby-Dick, edited  by Harrison Hayford, Hershel Parker, and G. Thomas Tanselle (Editorial Appendix, page 984).

Scholars agree, Melville was right on about Chase's 1821 Narrative having been ghostwritten.  Thomas Farel Heffernan proposed William Coffin, but Nathaniel Philbrick has suggested Coffin's son William Coffin, Jr. might be the one who ghostwrote for Chase.

"A graduate of Harvard College, Coffin, who was almost precisely Owen Chase’s age, would become a well-known teacher and temperance advocate on Nantucket, penning several pamphlets with stylistic similarities to the Essex narrative.  Thirteen years later, Coffin would be enlisted by Obed Macy to ghostwrite his magisterial History of Nantucket (1834).  It has been suggested by the Nantucket historian Helen Winslow Chase that Coffin was also the ghostwriter for William Lay and Cyrus Hussey’s Mutiny on Board the Whaleship Globe (1828)."  -- Loss of the Ship Essex

 In the preface to the book version of Israel Potter Melville claims about his version that

"... with a change in the grammatical person, it preserves, almost as in a reprint, Israel Potter's autobiographical story. Shortly after his return in infirm old age to his native land, a little narrative of his adventures, forlornly published on sleazy gray paper, appeared among the peddlers, written, probably, not by himself, but taken down from his lips by another. But like the crutch-marks of the cripple by the Beautiful Gate, this blurred record is now out of print. From a tattered copy, rescued by the merest chance from the rag-pickers, the present account has been drawn, which, with the exception of some expansions, and additions of historic and personal details, and one or two shiftings of scene, may, perhaps, be not unfitly regarded something in the light of a dilapidated old tombstone retouched."-- Israel Potter: His Fifty Years of Exile

As these two comments on ghostwriting show, Melville values fidelity to the original source.  Both narratives he supposes were transcribed from dictation by the principal actors, Israel Potter and Owen Chase themselves.  The same combination of appreciation for the firsthand account along with awareness of ghostwriting conventions is evident in "The Western Trail", an unsigned review of Thornton's Oregon and California in 1848: 

The incidents which we have glanced at in the aggregate will be found in Mr. Thornton's volumes related in a simple unaffected manner, though with little of the art of the trained writer. Yet upon the whole we would not have the book altered, though it were to pass through the hands of the most accomplished magazinist. Narratives of this kind are valuable, as they bear the authentic marks of the author's personality. We know, then, how to appreciate his facts—but let the same facts be related by a Captain Marryatt, or other adept in book-making, and we lose a proper guide to their valuation. -- Western Trail