Deception and Appearance vs. Reality

The book revolves around a murder mystery where characters, including the detective Collins, are not what they seem; the preface explicitly celebrates the reader's desire to be deceived, and the plot hinges on forged letters, hidden identities, and false accusations.

The wrong letter reveals hidden truths and challenges the characters' assumptions about the crime.

The murderer's phone call impersonates a friend, and the postmark on the envelope reveals that the crime was announced before it occurred, highlighting the theme of deceptive appearances.

The forged signature on the letter and the locked-room mystery suggest that appearances may be deceiving, as the characters speculate about hidden motives and false leads.

The letter reveals that Sir James was in grave danger despite appearing to go about his day, and the search of the room finds no secret exit, deepening the mystery of how the murderer escaped.