Scholars of both Franklin D.

Roosevelt and Winston Churchill know all about Harry Hopkins. He was the trusted advisor of President Roosevelt who was sent to Great Britain in January 1941 to assess the case for America joining the War. In poor health following surgery for stomach cancer, Hopkins was a hard drinker who looked older than his 50 years. Also, as an unelected advisor, he was hated by the rest of Roosevelt's cabinet for his influence over the President.

Sleep in Peace Tonight is a fictionalised version of his efforts to bring America into the War, efforts which took a great personal toll but gained him the admiration of Churchill, who later said of Hopkins that "his was a soul that flamed out of a frail and failing body".

Historically, MacManus's book has been scrupulously researched, and elements that have been invented have clearly been done so with the greatest of care. But Hopkins has been graced with a love interest in this version of events, in the shape of Leonora Finch, an English woman of great determination and resourcefulness who is willing to risk everything to serve her country. A fluent French speaker, she has been spotted by the secret services, and her apprenticeship in the dark arts begins with an assignment as Hopkins' driver on a fact-finding tour around Britain. Like so many others in this dangerous time, as the bombs fall around them, Hopkins and Finch find themselves irresistibly drawn together. The question they have to face is whether it's a wartime fling or something more permanent.

Leonora Finch may be an invention, but this novel is so deeply rooted in fact that it's an immersive panorama of bombed-out Britain. Hopkins becomes one of a small group of Americans, chief among them broadcaster and opinion-former Ed Murrow, who have grown to admire and empathise with the embattled Brits and have been persuaded that America should support them militarily. But the isolationalist lobby in the USA is in the ascendant, supported by the majority of the public, which has no desire to risk American lives so that Britain can retain an Empire of millions living under colonial rule.

To swing public opinion the opposite way would have been a Herculean task for anyone, let alone the sickly and exhausted Hopkins. In the end, as we know, it was Japan's attack on Pearl Harbour that brought America into the War; but that should in no way diminish his tireless work against insurmountable odds. Indeed, one of the most impressive aspects of MacManus's book is that it makes glaringly clear just how hopeless Britain's situation felt at the time. For those agonising months, Harry Hopkins was the only game in town, and he had to be courted.

The diplomatic game is played out in numerous conversations between Hopkins and Churchill, who bursts off the page and takes full-blooded life. The Prime Minister is indomitable, ebullient and utterly convinced of the rightness of his path, while also flawed, vulnerable and easily moved to tears. A force of nature with feet of clay, he's never seen without a glass of champagne, brandy or port, a facet of British politics which takes Hopkins aback and leads him to conclude that, in this country, alcohol is an essential weapon of war.

Roosevelt, though drawn from equally impeccable historical sources, makes a far less forceful impression. Afraid of being voted down in the Senate and losing in the court of public opinion, he comes across as a ditherer, a man who has to be forced by circumstances into making a decision. Still, there are some quite telling scenes between him and Hopkins, and tense exchanges with Eleanor Roosevelt, who believes that her husband's valued advisor has abandoned his ideals to side with warmongers and imperialists. One feels for Hopkins, reunited with his fiancée in Washington though his heart is 3000 miles away in London: with Churchill, whom he seems fated to disappoint, and the army driver with whom he's fallen in love.

Sleep In Peace Tonight is a satisfying wartime romance, but its real achievement is its superb evocation of Britain at its lowest ebb, a country staring into the abyss with only one slender thread of hope to which it can cling.