I’m a 12 year old, and I study with Franziska Huhn. I do study theory in my lessons, and I spend about 30 minutes per lesson. I don’t really know if what my teacher uses could really be classified as a book or not, because it’s a big binder full of papers on theory, and she also has some notes that she took herself when she was learning theory. I think that studying theory is useful, because it makes reading the music a lot easier, because you can recognize chords faster.