There are probably about 50 of them. Great novels, I mean. Works of fiction that stay with us forever. Even to name them is to summon up the feelings we had when we read them for the first time. They're books that, for the most part, we read when we were young, when we didn't know what our lives had in store for us, whether they would be defined by kindness or cruelty, success or the lack of it, love or the loss of it. There are different ones for different people. For me there's The Go-Between, The Cider House Rules, Wuthering Heights. For you there might be The Bell Jar, The Catcher in the Rye, Little Women. For all of us there's To Kill a Mockingbird.

A few months ago I was in Chicago, where I happened across a beautiful hardback edition of that novel in a local bookshop. Having not read it since I was a teenager, I bought it. At the till the cashier asked whether I was looking forward to the new Harper Lee novel, which had been announced some weeks earlier. I told her that I wasn’t sure, for there was something about the nature of its publication – buried away for years and rediscovered only after the passing of Lee’s centenarian sister, Miss Alice, who took care of her business and literary affairs – that bothered me.

It’s one thing, after all, for a manuscript to be published after an author’s death as a curiosity; it’s another thing entirely when the author is almost 90, living in an Alabama nursing home and possibly not in full control of her decisions. “Nelle Lee had a stroke,” said her minister, a longtime friend, in a newspaper interview earlier in the year. “She doesn’t remember anything; she’s essentially blind, profoundly deaf and confined to a wheelchair. You can draw your own conclusions.”

One of the joys of the earlier book is the way it uses the simple logic of a child’s mind to tear apart the segregationist policy of the United States in the 1930s. It’s hard to think of another novel narrated by as young a character as six-year-old Scout that is as authoritative in tone. The trial of Tom Robinson, accused of raping a white woman, is a farce; even when it is proved beyond doubt that he is innocent he is still found guilty, but when the mob arrives at the jail to lynch him it is Scout who shames them, with her questions about hickory nuts and the work of her father, Atticus, on one of the men’s legal “entailments”.

Go Set a Watchman takes place 20 years later, in the mid 1950s, when racial relations were, if anything, even worse, because the black community was rising up against its long-time oppressors and refusing, both literally and metaphorically, to sit at the back of the bus any more. It is into this atmosphere that Scout, now known by her more formal name of Jean Louise Finch, arrives, returning to see Atticus, whose health is in decline, and to flirt with Henry, her potential fiance.

The town of Maycomb, once so close to Scout’s heart, has become an alien place to her; she would lose her mind, we are told, were she to return to live there. She’s a New Yorker now but seems to be struggling in the city, for, on the rare occasions that she speaks of it, it is not with excitement or love. The confident Scout from childhood, that “juvenile desperado, hellraiser extraordinary”, has vanished; in her place is a rather morose young woman, floundering in life and struggling to reconcile her desire to marry with her emergent politicisation.

The truth is that Scout Finch is not a character in this novel at all; Jean Louise Finch is – and unfortunately Jean Louise Finch is nowhere near as interesting.

Before Go Set a Watchman was published this week there were whispers about how Atticus, that great moral compass in the courtroom, beseeching 12 white jurors to "review without passion the evidence you have heard . . . and . . . in the name of God, do your duty", had somehow become a racist. It seemed heretical even to suggest such a thing, despite the fact that there's a line in that same speech, which is often forgotten, when, speaking of Mayella Ewell, he remarks that "she did something that in our society is unspeakable: she kissed a black man".

One of the tragedies of Go Set a Watchman is the re-creation of that memorable scene where Scout sits in the gallery above the courtroom, watching her father speak so passionately. Now, however, when Jean Louise looks down at Atticus he is not defending an innocent "negro" but sitting next to one of his neighbours, "an ordinary, God-fearing man just like any ordinary man, who had quit his job to devote his full time to the preservation of segregation", and supporting him.

Her reaction is to believe that “the only human being she had ever fully and wholeheartedly trusted had failed her; the only man she had ever known to whom she could point and say with expert knowledge, ‘He is a gentleman, in his heart he is a gentleman,’ had betrayed her, publicly, grossly, and shamelessly.” We, the readers, know how she feels.

So, yes, Atticus has changed, although there is a later scene where he has a long and tedious argument with his daughter about why that is the case. But I choose to believe that this is not Atticus Finch at all, at least not the Atticus Finch that Harper Lee gave to the world. This is an early draft, a brush stroke of a character who would emerge differently when his proper voice and character were found. It is only because this novel takes place after the events of the first one that we feel let down by him. We shouldn’t; the younger Atticus is the one in whom the author asked us to place our trust.

Any number of inaccuracies are scattered through the book. We are told that Calpurnia, the Finches' maid, has "run off the place" after Jem's death; a few pages later she has grown so old "that she returned to the Quarters in honourable retirement". The title comes from a chapter of Isaiah, "For thus hath the Lord said unto me / Go, set a watchman, let him declare what he seeth", but the comma, which creates the rhythm of the biblical sentence, is missing here, giving a different sense to the phrase. But worst of all, unforgivable in fact, is that when reference is made to the trial of Tom Robinson the guilty verdict has suddenly become an acquittal, which changes the outcome of Mockingbird completely.

On one level this is a novel about disillusionment, the loss of childhood innocence and the importance of seeing our parents for who they are instead of creating idealised heroes. But the narrative is laden with so many boring flashbacks, endless discussions of the history of the South and intellectually sloppy arguments about race that one can understand why Harper Lee was advised to shelve the book and why, for so many decades, she wholeheartedly followed that advice.

Had this been published in the 1950s it probably would have caused something of a stir, but today it feels lifeless. If a stranger was to walk up to me on the street and say that everyone involved in ripping this manuscript from its vault, declaring that a frail old lady who never wanted to publish it was now somehow “delighted” by its appearance, and that those who stand to make extraordinary amounts of money by damaging a great writer’s legacy should hang their heads in shame, I would not punch him on the nose and demand a retraction. To my mind there’s something reprehensible about the whole exercise.

Consigned to the archives

Boo Radley, the mysterious and troubled neighbour who plays such a central role in

To Kill a

Mockingbird

, stays in his house for a reason. “It’s because he wants to stay inside,” Scout says when she comes to have a more nuanced view of her fellow townspeople. This novel is Boo Radley. It was meant to stay inside, locked away, hidden from the world. It was not supposed to be published. It was not supposed to be read. But when all the talk of the reclusive author has died down, and the inevitable articles about extraordinary sales figures have been consigned to the archives,

Go Set a

Watchman

will quietly slip away, not forgotten but little more than an interesting side note to a work of literary genius, the great American novel that has already been written and that, like our true understandings of Atticus, Scout and Jem, will survive forever.

John Boyne's first collection of short stories, Beneath the Earth, will be published by Doubleday in August