I've been reading a travelogue of a Japanese priest, Ekai Kawaguchi, who penetrated Tibet at the turn of the 20th century, and in his breakdown of life in Tibet he discusses the presence of Christians and Muslims in Tibet as part of the community in Lhasa. One detail that is mentioned almost in passing is that it appeared that much of the local Muslim population had essentially absorbed the Buddhist doctrine of rebirth into their beliefs. I've never heard of that mentioned before despite the role of Muslims in Lhasa being fairly well covered.
The book you're looking for is Islamic Shangri-la by David G. Atwill. The Tibetan Muslim (Khache) community dates from around 1653 when Lobsang Gyatso, the Fifth Dalai Lama, granted them land in Lhasa to build mosques and homes. The remaining Khaches mostly live in India, some in Kalimpong, some in Kashmir (where the term "Khache" comes from), and a small community in Saudi Arabia where they have built a successful community of hat makers.
The book doesn't go much into the theology or folk beliefs of Khaches. They do seem to have been some the historically most traveled Tibetans since they were merchants, well integrated into the nomadic communities, were among the first Tibetans to contact Europeans, and made regular pilgrimage to Mecca through India.
Atwill doesn't go over the theological beliefs of the Khaches, and the topic of the syncretization between Islam and Buddhism doesn't really enter into the conversation. But it's a good starting point. When Tibetan Muslims enter the historical record, their theology tends towards Muslim orthodoxy. And I'm not aware of any other deep penetrations into the folk beliefs of the Khache. My hypothesis is that Khache, being so well integrated into the Tibetan language community, share many folk beliefs, while their religious understanding fits well at home in the dar-al-Islam. The Muslim community in Ladakh, for example, has been known to house Buddhist monks in their homes under the belief that a Buddhist in the home brings good luck (a common belief among Buddhists), and they regularly feast the Dalai Lama's entourage when he visits. Ladakhi Muslims also celebrate Losar with Buddhists, and Christmas with Christians, while the Christians and Buddhists celebrate Ramadan with them.