THE LETTER OF MARQUE By Patrick O'Brian. Norton. 284 pp. $18.95 (Available in September)

THE Horatio Hornblower novels of the late C.S. Forester probably are inimitable, but that hasn't prevented numerous writers from trying. Some have produced what reviewers used to call rattling good yarns, but as any Hornblower devotee will tell you, that isn't enough. Forester's novels are romantic swashbucklers, to be certain, but that's only at one level. At another, deeper and more interesting, they are penetrating psychological studies of Hornblower in particular -- he is modeled loosely upon England's greatest naval hero, Lord Nelson -- as well as of various subordinates, Admiralty personages and other secondary characters; in combination with Forester's lucid and engaging prose, these studies give the novels a literary quality that few works in the genre have achieved.

This is the missing element in the imitations; it's also what Patrick O'Brian, in the third of his novels about Captain Jack Aubrey, comes closer than anyone else to duplicating without merely slavishly copying. The Letter of Marque is the first in this series to be published in this country -- its predecessors, Master and Commander and Post Captain, are being simultaneously issued in paperback by Norton at $9.95 apiece -- and it leaves the devotee of naval fiction eager for sequels. O'Brian manages both to satisfy the reader's longing for "more Hornblower" and to remain, throughout, his own man; that is no mean feat.

He establishes his distinction by the bold device of playing down the derring-do so beloved by so many readers and placing greatest stress on conversation, character development and byplay. Although the plot of The Letter of Marque includes two rousing engagements at sea, these are passed off almost as afterthoughts; one takes place entirely off-stage -- we learn of it only after the fact -- while the other, ostensibly the novel's climactic scene, is dispatched in a few well-wrought paragraphs. By contrast with the most dramatic of Hornblower's skirmishes with Napoleonic France, which Forester describes in ample and richly satisfying detail, these battle scenes are downright minimalist.

This isn't to say that O'Brian doesn't know his warfare -- to the contrary, he savors the minutiae of early-19th-century weaponry and can sling nautical lingo with the best of them -- but that he simply finds other matters more interesting. In The Letter of Marque the pivotal question isn't whether Jack Aubrey will be able to capture the frigate Diane from the French but whether he will be able to turn this engagement in his favor; he has been removed from the Royal Navy's roster of captains for a crime he did not commit, and has gone after the Diane in hopes it will prove "an action of obviously national importance that would justify a royal pardon or revision or restoration."

He undertakes the action as captain of the Surprise, a frigate once under his command but now sold out of the king's service; it has been purchased by his old friend, Stephen Maturin, "as a private ship of war, a letter of marque, to cruise upon the enemy." As a privateer he is entitled to take prizes; indeed early on he takes one sufficiently grand to set him up for life, but that is not his chief purpose. Not merely is he out to regain his unfairly lost honor, but he cannot imagine a life away from the sea, from command, from action.

He is a large, strong man, once "somewhat given to tears" in emotional moments but now made "as hard and dry-eyed as a man could well be" by the calumny of colleagues and the misjudgment of his superiors. The navy, once his home, now is the object of longing and anger; he means to exact his revenge, though that is not the term he uses, and to restore himself in the eyes of his family, his fellows and, not least, himself. THUS HIS story -- or, more accurately, this episode in his continuing story -- is not without its melodrama, both actual and psychological, but O'Brian declines to stoop to mawkishness or histrionics in the telling of it. From time to time his prose adopts the heroic style, but one never fails to sense an element of mockery just beneath its surface, especially when the singularly engaging Stephen Maturin is on the scene.

Maturin is a ship's surgeon, amateur naturalist and intelligence agent, though in this episode his role as spy is relatively insignificant. Here his chief obligation is to serve as underwriter for Aubrey's return to the wars and, no less significant, as a subplot all to himself. Part of this has to do with his infatuation with nature and science -- ballooning, now in its infancy, holds a particular fascination for him and others similarly inclined to practical inquiry -- but more of it has to do with his complicated relations with his wife, relations that have been as much damaged by misunderstanding as has been Jack Aubrey's naval career; the encounter with his wife that is the novel's concluding event is notable for the wit, ingenuity and charm with which O'Brian brings it off.

In this as in everything else, he has a fine eye for nuance and detail. Early on there's a lovely little scene in which Maturin, making his way on foot to meet Aubrey at the seaport of Shelmerston, is waylaid by a fellow naturalist who tempts him with a sighting of bustards (Otis tarda), game birds whose charms Maturin simply cannot resist; the subsequent delay in his arrival is, Maturin tells Aubrey, due to "a gross self-indulgence in bustards." As for Shelmerston, O'Brian's account of that nautical town is vivid and affectionate; when Aubrey's wife briefly establishes residence there, we see it through her eyes and the view is especially agreeable.

All in all this is fine stuff, and the promise of more of it in the future is entirely inviting. As is to be expected, O'Brian's publisher compares him to Forester -- carrying matters a bit far, he makes the comparison invidious to the latter -- but in the end that is neither here nor there. It is good to have someone working Forester's old territory, but what's even better is that he works it in his own way and works it well.