After last month's encounter with DH Lawrence, it's a relief to come to a book that is as easy to like as it is to admire. Easy, in fact, to love. I fully understand why reading group contributor babytiger might give his son the middle name "Atticus" and why he describes To Kill A Mockingbird as "an insanely good book".

It was many years since I'd last read the novel when I took it up again, and all I really remembered was my affection from first time around, alongside a few sketchy memories of sunny Southern weather, childish games and a bug crawling out of someone's hair. But it didn't take long before I realised why I enjoyed it so much all those years ago. It took, in fact, two pages of pleasant reading and then the following:

"Maycomb was an old town, but it was a tired old town when I first knew it. In rainy weather the streets turned to red slop; grass grew on the sidewalks, the court-house sagged in the square. Somehow, it was hotter then; a black dog suffered on a summer's day; bony mules hitched to Hoover carts flicked flies in the sheltering shade of the live oaks on the square. Men's stiff collars wilted by nine in the morning. Ladies bathed before noon, after their three o'clock naps, and by nightfall were like soft teacakes with frostings of sweat and sweet talcum."

I'll buy that over every single description of Paul's ecstatic flower-sniffing in Sons and Lovers. I'll take it over the whole novel, in fact. To explain why is to try to explain magic. Sure, there's technical prowess in the imagery (how perfectly all that sagging and wilting leads up to the teacakes) and the choice language (notice they are "ladies", not women; look at the hiss and stick of the sibilance, and all those letter "g"s in the muddy second sentence). But really what matters is more ephemeral, an alchemy of mood and voice that has lulled millions over the last 50 years. So many millions, and with such delight that, Stephen Metcalf noted in Slate, the novel "has become an inescapable fact of America's civic religion". It's a book held in such reverence all over the English-speaking world as well as the USA that to criticise it is almost a form of blasphemy. That hasn't stopped Metcalf and a few other critics having a go, however.

The most famous attack came from that other great Southern US prose stylist, Flannery O'Connor. When the book was already selling truckloads in the early 1960s, she observed: "It's interesting that all the folks that are buying it don't know they are buying a children's book."

Ouch! Although, is that really something to worry about? Surely part of the appeal of To Kill A Mockingbird is its ease, its apparent simplicity and its delight in childhood adventure. Personally, I have no objection to reading any kind of children's book: so long as it's as good as this one.

More damaging are a series of attacks begun by Thomas Mallon in the New Yorker in 2006. Mallon went first for the voice that most people find so beguiling: "Lee's narration remains a patchwork, mixing an adult's and a child's perspective according to no logic other than the immediate exigencies of the plot." There isn't much defence here. When Scout is six, she often sounds as though she could be 16. When Lee does remember to relate events from knee-height, she often punctures the illusion by interjecting variations on the "as I look back on it now" formula into the text. The only real answer to Mallon here is to shrug the shoulders. If I stand far enough back from the text, I can see a few rough edges – but when I was reading, immersed in Scout's world and within the logic of the plot, I didn't care. If you fall for Scout, as most people have, you can forgive such lapses. You probably won't even notice them.

Mallon also claimed, in a lovely phrase, that the novel is "moral Ritalin". Lee has "allowed the war between good and evil to be … a simple matter". Atticus is a "plaster saint", and if there are shades of ambiguity, the book never persists in them: "Mr Underwood, a man who 'despises Negroes' but protects Atticus with a shotgun, is glimpsed a couple of times and then dropped." Here I can defend Lee. Even Atticus, delightfully good as he may be, has flaws. He allows Aunt Alexandra to override his better judgment, and he also occasionally employs the very hierarchical language that the novel sets out to criticise: "you grow older, you'll see white men cheat black men every day of your life, but let me tell you something and don't you forget it – whenever a white man does that to a black man, no matter who he is, how rich he is, or how fine a family he comes from, that white man is trash." Of course, you might read that as a noble sentiment. It isn't wrong, cut and dried. But we should credit Lee with some ambiguity here.

Meanwhile, the more interesting moral complexity comes on the other side. The good people in Scout's world are very good – but even the worst are granted motive and treated with empathy. Atticus's frequent request that Scout picture herself in the shoes of her adversaries sometimes seems like hokey, homespun wisdom, but adds more complexity to the novel than Mallon grants it. We are even asked to understand why someone like Walter Cunningham might join a lynch mob. Harper Lee may draw a thick strong line between right and wrong, but she allows for the fact that people will wander either side of it. It isn't so "simple" as Mallon claims.

Meanwhile, back on the subject of Atticus's wisdom, Allen Barra in the Wall Street Journal followed Mallon up by calling out the great lawyer as "a repository of cracker-barrel epigrams". Atticus, he said, "speaks in snatches of dialogue that seem written to be quoted in high-school English papers".

He illustrated this with the following:

"The one thing that doesn't abide by majority rule is a person's conscience."

"Why reasonable people go stark raving mad when anything involving a Negro comes up is something I can't pretend to understand."

"If you can learn a simple trick, Scout, you'll get along better with all kinds of folks. You never really understand a person until you consider things from his point of view …"

Bad Atticus! Bad Harper Lee! Too eloquent by half. Winston Churchill had similar issues. Hamlet too might have been purpose-built to give sixth-formers something to spout back at examiners. But though Atticus may seem to state the obvious now, in 1930s, or 1960s, or even today's Alabama, plenty of people would surely find plenty of his statements, shall we say, difficult to stomach. This, after all, is a book that helped to change the world. That perhaps shouldn't change the way we view it as literature – but is testament to its power. Would To Kill A Mockingbird have mattered so much if it was badly written? Maybe. Anyone who's struggled through Uncle Tom's Cabin will know that awful writing can still strike a chord. But it surely helps that Harper Lee plays her readers so well.

Elsewhere … Regular readers of the Reading group will be pleased to see that it got a review. From Howard Jacobson! He rather disagrees with my assessment of Sons and Lovers. It's pretty thrilling reading.

• Also, if you'd like a shiny new edition of To Kill A Mockingbird, we've got some copies to give away. If you're a UK reader and would like one, post a comment with an "I want please" somewhere. The first 10 to make a claim will get a copy. But don't forget to email Ginny.Hooker@guardian.co.uk to claim yours – we can't track you down ourselves.