In the stage adaptation of To Kill A Mockingbird at London's Barbican, Harper Lee's novel is the star of the show. The cast hold up battered old copies of the book. They read the opening paragraphs aloud; they chalk on the stage a town plan of Maycomb as it is described in the text.

The message is clear: we are about to make a journey to a enchanted world, and the way into it is not through the back of a wardrobe or down a rabbit-hole, but via a route accessible to anyone: through the pages of a book.

As grown-ups, we read for all sorts of reasons: for entertainment, for instruction or to stop ourselves from thinking. Children, by contrast, read in order to become. In the books we read when we are young, we are not reading about the Railway Children, Harry Potter, Jo March or Lee's heroine, Scout Finch. In the space between two book covers, the boundaries between reality and imagination dissolve and for a long moment, you and the characters are one. Afterwards, you aren't quite the same person as before. Among all the other formative influences of childhood there remains a tiny bit of you that is forever Harry, Jo or Scout.

Even for a grown-up, the sensation of reaching the final page of a captivating book is that of sharp bereavement. To know that you will go no more a-roving with Dorothea Brooke, Elizabeth Bennet or, to take a current favourite, Lena and Lila - the heroines of Elena Ferrante's masterly Neapolitan tetralogy - to know that what you know of them now is all that there is to know, feels like an affront.

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So if the chance were offered to continue an enchanted journey with a beloved childhood companion - well, you'd take it without hesitation, wouldn't you? Which is why, millions of people who read Mockingbird when they were young will go straight out to buy Harper Lee's only other full-length work of fiction. And what they discover will make many of them sad.

You don't have to read very much of the first chapter of Go Set A Watchman to see it contains only a faint shadow of Mockingbird's originality and charm. Finished in 1957, three years before Mockingbird was published, and narrated in the third person, it describes the adult Scout's return to Maycomb from New York, where she now lives. The Atticus of Watchman is not the righteous authority figure beloved by generations of readers, but an ageing, arthritic racist.

The unmistakable mark of a great book is the feeling of absolute trust that it inspires in its readers. Mockingbird gives you that confidence by the time you have reached the end of the first page. Watchman reads like a young writer's first, hesitant pass over material that will later be transformed into a masterpiece by tough editing and hard work.

For literary scholars, everything a writer produces has its own particular charm. For the rest of us, common readers, who open Watchman hoping for enchantment and find it absent, the publication of Lee's "lost" first novel is a reminder that writing a masterpiece is not an act of magic, but a grim slog of failing, trying again and failing better.

Go Set A Watchman
by Harper Lee
(William Heinemann $50, hardback)

- Canvas, Telegraph