MIDDLE PASSAGE By Charles Johnson (Picador, $114) FOR Rutherford Calhoun, a recently-freed slave in the American South of the 1830s, New Orleans is Paradise. Its crowded waterfront bars and accessible brothels are the perfect playground for the ex-Illinois farmhand with a gift for talk and hunger for life. But Paradise is about to turn on Calhoun. Forced to run from the clutches of a woman in love and a growing number of shady creditors, Calhoun makes his escape by stowing away on The Republic - only to find the ship is a slaver bound for the west coast ofAfrica to pick up another human cargo. Charles Johnson's Middle Passage is an ambitious undertaking, a horrifying look at the human condition that motivated the slave trade and an exploration of the contemporary dilemma of black America. Mr Johnson writes with great humour, and in his ''hero'' Calhoun he creates a rich character - a street-wise scheming but happy-go-lucky opportunist on the one hand and a witless innocent on the other. On board the slaver Calhoun embarks on a voyage of self-discovery. As the only black member in the ship's company - a motley bunch of degenerates - Calhoun is mistrusted from the start. Once the ship takes on its human cargo of 40 Allmuseri tribesmen, Calhoun becomes a go-between for the slaves and crew (though accepted by neither group) and is confronted head-on with his own rootlessness. The ugliness of the eventful voyage home is relentless. Through the eyes of Calhoun, Mr Johnson strips away one layer of evil from his characters, only to leave readers in horror at an even darker side of the human condition. Mr Johnson has clearly taken the time to research this book extensively - his descriptions of shipboard life are gritty, if not downright dirty, and there is little of the gloss that is usually associated with the sailing ships of this supposedly romantic time. Middle Passage is a provocative novel although sadly its ending is a little flat. Having journeyed so far, the reader looks to the author to provide a meaningful explanation for the dark events Calhoun has been forced to struggle through. But he prefers to leave that task to his audience.