Two Great War aviators, Alcock and Brown, flew from Newfoundland to Ireland in 1919.

They carried the first transatlantic airmail, a peaceable thing to do. As Colum McCann writes, they were "taking the war out of the plane, stripping the thing of its penchant for carnage". Tucked in Brown's coat was a private letter, given to him by a girl, from her mother to a family in Ireland. TransAtlantic is about the fortunes of the letter and the people connected to it.

It's also about the exchange of culture between North American and Irish civilisation over the last century and a half. There is the striking story of Frederick Douglass, the escaped American slave who became a writer and a giant figure in the abolitionist movement. In 1845, he visited Ireland on a lecture tour. The potato blight had just started. I must say, it is rather fascinating for a reader in Scotland now to watch Douglass's descent from Dublin into the regional strongholds of the English rulers of Ireland: he didn't like it. The roads were full of mortally ill people, the English Army savage in its attempts to keep the peace.

The English "were melodic and well informed, but when he asked of the hunger that he had seen in the streets they said there was always a hunger in Ireland. She was a country that liked to be hurt. The Irish heaped coals of fire upon their own heads - it had always been so".

Lily, a servant who encounters Douglass, is inspired by him to go to America. Her disappearance prompts a search for her, in one of many historically evocative scenes: "The closer they got to the sea, the more the roads thickened with leaving. Vendors had set up stalls against the hedges. Families were hawking the last of their possessions. Douglass and the sisters had to slow their horses down to get through the crowds.

"All manner of things for sale. Fiddles, inkwells, pots, hats, shirts. Paintings strung on the hedges. Curtains hung from the branches of trees."

Douglass has tried to tell himself that the Irish aren't as oppressed as American slaves, but in the wake of Lily's departure, her utter poverty, he sees that, practically, they are. There are invigorating ideas about oppression and freedom throughout the novel.

The other major historical figure is that of Senator George Mitchell, the American who brokered the peace talks in Northern Ireland; so Americans wonder how to help Ireland, while Irish men and women wonder at America.

The description of Mitchell's brutalising schedule, the minutiae of diplomatic work, are very effective: "The British and their words. The Irish and their endless meanings." There is a lot of tea, but McCann is one writer, maybe the only one, who can handle tea: "This whole memory, it will taste of tea.

"He has become a man of tea. He never would have believed it." There's also a telling passage in which it develops that Tony Blair is the only guy who knows where there's a shower in Stormont Castle. Guess he was determined to look better and smell better than anyone else at that table.

A quiet young man is killed, for his bird gun, by a paramilitary group. He is effectively the last in the line of the people the novel is about, and the story should end there.

Instead, we're given an uncharacteristically plain account of the transatlantic letter. It turns out not to be very interesting. It has bounced around the novel's characters for 100 years, and the reasons given for its never having been opened are mumbo jumbo. It's a kind of botched MacGuffin.

Writing suddenly in a different voice, McCann's invention and mellifluousness desert him.

He is a subtle writer, and TransAtlantic is an engaging kind of historical fiction: intimate and fresh, with not a trace of the lamp in the air.

He is far better at the oldest settings in the story, however; Douglass's tour, the description of the transoceanic flight, and the story of Lily in America are more engaged, warm and inventive than the modern sections. As with most good writers, he's best at what he knows nothing about.