Coming-of-Age
Like many children's novels, this book is a bildungsroman, or coming-of-age novel. Harriet grows and matures over the course of her story, learning important lessons about kindness and humility and figuring out how to solve her own problems without the comforting presence of her beloved Ole Golly. Having her notebook stolen challenges Harriet in many ways, but more importantly, it gives her the chance to prove that she has grown up into a responsible and thoughtful young lady.
Different Ways of Living
On her spy route and in her everyday life, Harriet encounters all kinds of people who live in very different ways. She herself comes from a wealthy, comfortable family, while her friend Sport has to take on a lot of adult responsibilities and work harder to make ends meets because of his father's negligence. Mrs. Golly is content to live alone and uneducated, and Mr. Waldenstein has renounced all his material wealth to be a delivery man. This book sends an important message that life looks drastically different for different kinds of people, and it is important to expose oneself to all kinds of different ways of living in order to avoid ignorance and gain good judgment and sympathetic understanding of others.
Isolation
Harriet feels very isolated. This is partly because her parents are not very involved in her life, and partly because she is observing everyone around her in order to write about them, which puts her in the position of being on the outside looking in. Once her friends discover her journal, she is further isolated, as they all turn against her and the journal is her only confidante that remains.
Friendship
At the end of the book, Harriet realizes how important her friendship with Sport and Janie is to her, even though she had previously attempted to convince herself that it did not matter. With Ole Golly's advice, Harriet learns that sometimes prioritizing the relationships in her life means that she must lie and apologize, even if she does not feel like she was wrong, because making others feel good is an essential part of being a good friend.
The Truth
As a budding spy, Harriet is constantly on a hunt for the truth. She aims to know everything there is to know, and record it all in her notebook so she can always remember it. However, through the experience of losing her notebook, Harriet learns that the truth is more complicated than it seems. The truth can hurt people, and, in certain situations, it is unwelcome. While seeking truth is a noble cause, the truth must be handled carefully and in a way that does not hurt others.
Bullying
When her journal, with its often-harsh judgments of her friends, is discovered, Harriet is viewed as the "mean girl." But although her journal entries can be seen as mean-spirited, Harriet never intended anyone to read them and so was never a bully in the conventional sense. On the other hand, her classmates' feelings are hurt after they read her observations about them. The way they react is definitely bullying: ignoring Harriet, saying mean things so that she can hear them, and throwing spit balls. The book returns to the theme of bullying many times and throws up an interesting question when it comes to deciding whether Harriet is a bully or a victim of bullying.
Being True To Oneself
Many of the characters in the book are a little outside of the norm, and the book shows that this is something to celebrate, not something to be ashamed of. One example of this is Sport, who shares the household chores with his father since his mother left. Another is the boy with the purple socks, whose fashion choices are unusual but who enjoys being different. The characters in the novel are all very individual but this is always seen as a positive thing.
