This sentence is included in "Glory and Contribution", which is a famous saying of his scholarship. It aims to encourage people to study more without fear of difficulties, and only hard work can lead to success. These two sentences use metaphor, duality and intertextuality.

"Book mountain" means that books (knowledge) are like mountains, and "learning the sea" means learning as deep as the sea.

"There is no limit to learning the sea, and there is no limit to learning the sea." It means that knowledge and learning should be conquered by diligence and hard work, such as the bright moon in Qin dynasty and the customs in Han dynasty. This is intertextuality.