The Beginning of Frankenstein

I wasn't very excited to say the least to have to start reading a new book in AP. That class is already stressful enough, and adding another thing that is required for us to do does not help my situation out at all. AP this year is not as fun this year as I had thought it would be. 

   

   Starting Frankenstein was interesting to say the least. It had started out with these letters sent from some guy to his sister maybe? And then the main character of the novel and his story begins right after. It was all very confusing to me. After I had finished reading the part required, I thought that I had understood what was going on. Then we came into class and took the quiz that our teacher had made for us and I was so lost.... I had no idea what was going on. 

   Who writes a book with letters at the beginning that aren't even from main characters in the book? It doesn't make sense and I'm sure my grade showed that..