Are there any theoretical books about calligraphy or something?
This is too much! There are ancient and modern ones.
Sun Shu Pu in Tang Dynasty, Shu Duan in Tang Dynasty, Shu Fu by Dou Meng, Shu Duan Xu in Song Dynasty, Yi Tan by Bao in Qing Dynasty, Guang Yi Tan by Kang Youwei, Modern Qigong Tell You Calligraphy, Qiu Zhenzhong Calligraphy-