Until recently, Marilynne Robinson shared something in common with JD Salinger and Harper Lee, namely an immense reputation as a novelist gained from a single published novel. Housekeeping, published in 1980, is a remarkable work, a slim, haunting story about the internal world of two children and the eccentric aunt who looks after them. It is unostentatious, but nevertheless gives off the unmistakable scent of genuine art.

For a quarter of a century, it seemed as though Housekeeping was to remain the author's sole, enigmatic fictional statement, but in 2004, Robinson published Gilead. Set in a small Iowan town in 1956, it takes the form of a letter from a dying, elderly preacher, John Ames, to his young son. Ames is a monument of goodness, a civil and intelligent man whose final years have been illuminated by a wife several decades his junior and the child they created. His only worry, as his selfless life edges towards its conclusion, concerns the return of Jack, the prodigal son of his best friend, the Reverend Robert Boughton. Having fled 20 years ago after fathering an illegitimate child, Jack has reappeared and has got himself on good terms with Lila, Ames's wife.

Home, a companion piece to Gilead, moves across town to the Boughton family at the time of Jack's arrival. While it once heaved with eight children and two parents, the house now contains only the widowed and the ailing Rev Boughton and his daughter Glory, who, approaching middle age, has come home following the collapse of her engagement to a man who turned out to be already married. Glory is a straightforwardly saintly woman; when the brother she hardly knows returns, following a break-up of his own marriage with a churchgoing woman named Della, she is unnerved but kind, looking after him untiringly and even managing to hold her tongue when her father fawns over his son and treats her, at times, like a servant.

The Boughton men are less easy to label. Jack, although possessing an unwholesome reputation, is softly spoken, never one to shout or brawl. He is diffident and polite and it is only his haunted appearance and the history of childhood misdeeds that mark him out.

His father is similarly contradictory. For 20 years, the Rev Boughton longed for his son's return. He sent him money whenever he could track him down and, one could almost say, cherished him above his other children, partly because he believed he had failed him. But, in spite of what he claims, the Rev Boughton cannot forgive Jack for anything: forgiveness is merely a doctrine he aligns himself to in conversation, so as to appear worthier than he actually is. In truth, he is full of anger towards Jack; this emerges in bursts that soon accumulate into an assassination of character which his son scarcely deserves.

One of the cleverest things about Home is its interplay with Gilead. In that novel, the honourable Ames admits that he is awed by Jack, whom he considers icy and uncomfortably perceptive. In Home, which is told in the third person but from a Boughton perspective, we see Ames as an angrier, less charitable man, and we know that Jack is actually nervous of him and desperate for his approval. One can read about the same encounter in both novels and see that there is a misunderstanding at large: Jack doesn't want to steal Ames's wife, but simply wants to be liked by Ames, a man whom his father worships; Ames, in contrast, appears terse with Jack only because he is frightened of what he perceives as his corrupting influence.

Things sag somewhat in the middle of the novel, before gaining momentum in the tumultuous final quarter. There are also two other problems: why is Jack so convinced of his own enduring badness, when there is little evidence of it, and why can't he go back to Della, when it seems to be what both parties want? However, while the earlier Gilead is the more faultless novel, Home is in some ways the more interesting, since it deals in grey areas rather than in simple decency. Nothing is what it seems: polite exchanges are laden with wariness and irritation; and the supposedly loving reunion between Jack and his father is always governed by a past that can never be forgotten or even properly understood.