Which two books were put forward by Ruan Yuan?

He put forward the famous "On the North Monument and the South Post" and "On the North-South Calligraphy School". The development history of calligraphy is recorded in both books in great detail, which can be compared with Dong Qichang's view of "North-South School". Ruan Yuan's calligraphy creation is far less than his theoretical achievements, but his painstaking research on stone tablets is impressive and his style is extraordinary, which is above modern calligraphy. Its official script imitates Yi Ying Bei and Tian Fa Shen Bei, flying freely, and its running script also contains the legacy of the Northern School, which is cool and refreshing.