Intelligence gathering and intelligence analysis are distinct and complementary skills. It is not enough for agents to collect informative material if there are no desk officers able to interpret the data. The Secret Twenties exemplifies the perils of a mismatch between copious, accurate information and less reliable analysis. Timothy Phillips has delved into the MI5 files of the 1920s that have been released to the National Archives. These files are a wonderful source of social history, he has found, because they contain, among other things, correspondence intercepted under Home Office warrants. His beavering has produced good stories, with colourful personalities and a great deal of derring-do. He is an accomplished raconteur who tells his tales with a brisk pace, a sharp eye for detail and a sense of absurdity. His lively enthusiasm about his discoveries makes this book a rattling good read. But the inferences he draws from the evidence and the overall interpretation that he offers do not match the quality of the storytelling.