It's been years since I read any Pepys, and I'm not sure what he has to do with WWI, but I can say that diaries from any time period are a great indicator of what was REALLY going on. Historians are usually concerned with dates and locations and who won and who lost, so the daily lives of the people tend to get buried under the "important" stuff. But it was the people who lived through it, and their stories are equally important.