No. There are two sequels to Harriet the Spy (well, two written by Louise Fitzhugh, The Long Secret and Sport), this is neither of them. It was published too late to be any of those (and two early to be the sequel written by somebody else, Harriet Spies Again). Also, Harriet never got involved with government things, and lived in NYC :)

But thank you.

Edit: Sport is definitely the also-ran of the original sequels, btw, and was out of print for a very long time, but you can usually find a copy of The Long Secret if you try.