The success of The King's Speech helped Peter Conradi's rush-released book of the same name top the charts in 2011. His new book sets out the events portrayed in another biopic, Hyde Park on Hudson, set around the picnic that President Franklin D Roosevelt hosted for George VI and Queen Elizabeth during their three-week tour of North America in 1939.

At stake in the visit was geopolitics – Britain wanted to coax the United States into the brewing conflict in Europe – but what fired the public imagination more was a perceived clash between national stereotypes of British reserve and backslapping American cheer, symbolised in a question of culinary etiquette: would FDR serve his guests hot dogs?

Roosevelt's wife, Eleanor, wrote that "there are few individuals in any walk of life who are not thinking and wondering about the King and Queen of England", and you have to share that goggle-eyed view – widespread in the diaries and newspapers that Conradi fillets – to get much out of this book. How important is it to know that on a given day George wore "a blue-grey lounge suit" before changing into "a grey flannel suit" and later wearing "a double-breasted dinner jacket"?

There is little attempt to analyse or help us make sense of the mania that apparently led a crowd of 100,000 New Yorkers to overwhelm 14,000 police when the royals came to town. Perhaps it's because, at £20, the fortunes of this slim volume may depend on that kind of devotion.