Harriet the Spy holds up remarkably well for a book published in 1964. It's genuinely funny, the premise is timeless (what kid doesn't want to be a spy?), and Harriet herself is a fantastic character—stubborn, observant, ambitious, and real.

The book doesn't pull punches about friendship conflict or the consequences of cruelty, which makes it far more valuable than sanitized stories where everyone learns to share nicely. Kids see Harriet mess up badly and have to figure out how to repair the damage, which is exactly the kind of emotional learning middle-graders need.

The 1960s setting is noticeable but not a dealbreaker—modern kids might need a quick explanation of what a 'dumbwaiter' is, but the core story transcends its era. This is one of those rare classics that actually deserves its status and won't bore contemporary readers.