The Chinese government is known to have strong hands on any media. Books, magazines and CDs can be confiscated if the content is considered inappropriate, although customs usually doesn't bother to take your English books away, if there are no explicit photos depicting politics of China.
No so-called Anti-Chinese materials: Tibetan Lion-Mountain flag, Falungong, Taiwan national flag.
Books: any books with photos on Dalai Lama or Tiananmen Square incidents. Expect a questioning session if you bring a book with Chairman Mao's portrait. George Orwell's novels Animal Farm and Nineteen Eighty-Four have been seized at Chinese airports as they are critical of communism.
Pornography: Heavy penalty is imposed on all pornography and penalty is counted based on the number of pieces you bring into the country. If they consider what you bring is too much, let say, more than 100 videos on your laptop, they will likely detain you.