I first read Harriet the Spy
when I was about eleven years old; the same general age as the title character. I remember that I quite enjoyed the book, and I'm fairly certain I spent a good several months trying to be a spy just like Harriet. But beyond that I had very little recollection of the story.
The story holds up fairly well, even given the huge technological advances that have been made since it was written in 1964. I actually think the most telling thing as to the age of the story was the fact that Harriet's class at school had ten students. That class size seems more or less unheard of these days, even in private schools.
With my most recent reading--nearly eighteen years later--I found myself more interested in the behavior of the people Harriet observes on her "spy route" than I was in her antics. It was interesting to see how they were described, and the little hints and clues as to who they might be (outside the rather limiting filter of an eleven-year-old's perspective).