And when they ask us how dangerous it was . . .

As every biographer knows, all evidence is suspect. Probably the diary comes nearer to the truth than any other source: it is subjective and no doubt biased but a least it usually reflects what the author really thought at the time. Letters are second-best. They too are contemporary but they contain what the writer wanted someone else to think, not necessarily what he or she thought themselves. Most problematical of all is oral testimony.

Memory plays fearful tricks. With the late Tom Harrisson I once conducted an experiment. From the diaries kept by Mass Observation volunteers during the second world war we picked a few which contained particularly vivid Blitz experiences. We then wrote to the authors, asking them, without referring to anyone or any document, to tell the story again. Half a dozen obliged. Any resemblance between the original story and the version recounted 30 years later was almost entirely coincidental. They got everything wrong: time, place, sequence of events. In almost every case they moved themselves closer to the centre of events; what had happened to a neighbour now happened to them.

Both these books are based on oral testimony. The first, The World at War, skilfully edited by Richard Holmes, draws on the vast pool of interviews given for the 1973 television programme of the same name. The second, Young Voices, is a pot-pourri of childish or teenage recollections of the second world war. Given the reservations above, can such books be worth anything? The answer must be, yes, a great deal. Nobody in their senses is going to scour these books for precise information about the course of the war. ‘The great virtue of the material,’ writes Richard Holmes, ‘lay not in its narrative detail but in its impressionistic quality.’ As a picture of how people felt, of the atmosphere of the times, these books come as near the truth as one can get.