What are "hard books" and "soft books"?

"Hard books" are heavy, profound and thought-provoking. Later, it was said that we should not only read "hard books", but also read several "soft books" to adjust, that is, those that are more relaxed, pleasant and emotional; I also feel very comfortable in this way. Hard books help us understand things, increase our wisdom and open the door to thinking, while soft books make us more comfortable in our daily life, make our emotions yawn and relax.