Hi there teeny and welcome to the site!
I'm afraid your library and the other sites are quite correct. Wee Free Men is the first of the Tiffany Aching books. The Amazing Maurice and His Educated Rodents is not a Tiffany book. It's set on the Discworld, but very much in another part of that planet.
I Shall Wear Midnight is going to be the fourth in the series, but as you point out, it hasn't actually been published yet. In fact it may not even have been written.
I'm not sure if you've read the books yet, but I'm sure you'll enjoy them. The Amazing Maurice won the Carnegie Medal and is a stand alone book.