One of the mysteries that continues to puzzle historians is why, at the height of its global power and influence in the 1400s, China chose to turn inwards and away from the rest of the world. Nicholas Kristof provides a good overview of this question in 1492: The Prequel.
Kristof gives a great deal of attention to the great fleet of the Chinese admiral, Zheng He. A good profile of Zheng He is Admiral of the Western Seas. One historian argues that Zheng He reached America 72 years before Columbus did.
An interesting fact about Zheng He is that he was a Muslim. This aspect of his story is given prominence in Zheng He: The Chinese Muslim Admiral.
Muslims have been in China since at least the 700s C.E. and there are approximately 20 million Muslims in China today. Historical Sketch of the Hui Muslims of China provides some information.
Many Chinese Muslims are from the Xinjiang province. As its former name of Eastern or Chinese Turkestan indicates, Xinjiang is closely related to the Central Asian republics such as Uzbekistan, Kazakhstan, Kyrgyzstan, and Tajikistan. The major ethnic group are the Uighurs.
Xinjiang did not come under effective control of China until 1949, and has been struggling since then for independence. As Human Rights Watch reports:
Much like Tibetans, the Uighurs in Xinjiang, have struggled for cultural survival in the face of a government-supported influx by Chinese migrants, as well as harsh repression of political dissent and any expression, however lawful or peaceful, of their distinct identity. Some have also resorted to violence in a struggle for independence Chinese authorities have not discriminated between peaceful and violent dissent, however, and their fight against "separatism" and "religious extremism" has been used to justify widespread and systematic human rights violations against Uighurs, including many involved in non-violent political, religious, and cultural activities.Additional reports on China's repression in Xinjiang and of Chinese Muslims are Strangers in Their Own Land and Muslims refuse to be pushed out of Beijing. Added: You can also read about the plight of Uighurs in neighboring Kyrgyzstan. On a brighter note, you can read about Sufi orders in China. Japanese Muslim author Sachiko Murata has written Chinese Gleams of Sufi Light, a translation of two Chinese Sufi books and an examination of their historical and religious context.