This is an extremely interesting crime story set in the 1930s; the mystery is absorbing, the writing excellent and the characters interesting and likeable. The inclusion of real-life '30s detective writer Josephine Tey, about whom we convieniently know very little, so the author can take whatever liberties she likes with her life, makes it even more charming and the fact that it is not her that does the detecting helps to keep it realistically plausible. The portrayal of the "lost" generation, which came out of WW1, and how that war affected the mindset of an entire country takes it to a whole other level, as the book becomes worth reading in its own right, regardless of the mystery angle.
My only possible objection is that it doesn't seem to stay true to the period in certain aspects. For example, homosexuality certainly existed and I am willing to believe that in the theatre world it was even more or less excepted, but the idea that a homosexual couple would kiss in public, when homosexuality was an actual crime, is a bit of a stretch. However, anybody but a contemprary writer is bound to get at least a few things wrong, so this does not detract greatly from the novel.