What is the title of this children's book?

I am looking for a book that I read as a child but do not remember the title or author. It was in the mid to late 90s. It was about a courageous mouse who had a knack for spelling. I think there was a dragon the mouse fought. For some reason I keep thinking of a word that starts with "I" and may have had 11 letters in it... maybe it was a magic word in the book. If I remember correctly the cover of the book had the mouse on it, holding a sword perhaps, with a deep red border.