Legal basis: Criminal Law of People's Republic of China (PRC).
Article 218 Whoever, for the purpose of making profits, knowingly sells infringing copies specified in Article 217 of this Law, and the amount of illegal gains is huge, shall be sentenced to fixed-term imprisonment of not more than three years or criminal detention and shall also, or shall only, be fined.
Article 2 17 stipulates that the fourth item of infringing copies is "books with exclusive publishing rights enjoyed by others", which is what we usually call pirated books.