I never read any of those books on my own but my mom and sisters read The Ramona books together, and probably some of the others, and I often listened. They read them multiple times so I probably heard the books all the way through, but not necessarily all together in one go (which feels weird in retrospect but I didn't think so at the time). I think I also had teachers read a couple of them to my class. I also remember the TV show, which was a faithful adaption of bits and pieces of various books.
I don't know anything about Fifteen. Looking at the characters listed on the Wikipedia page, it doesn't look to be part of the main Beverly Clearly Literary Universe.