Reincarnation (the idea that all living things die, then return to life in a new form) is central to Tibetan Buddhism. Tulkus are believed to be the only living beings who are able to know and choose in advance the form and manner in which their spirit will be reborn on Earth. In addition to being reborn many times, and accumulating wisdom in the process, tulku and other Tibetan holy men are also believed to be ongoing, earthly manifestations of bodhisattvas. (Bodhisattvas are semi-divine beings, who symbolize and teach Buddhist spiritual values; they are roughly equivalent to major saints of the Catholic church.)
'Tulku: A Tale of Modern Ninja' is also a very cool book, written by Stephen K. Hayes in 1985 (Contemporary Books, ISBN number 978-0809253326). It is a kick-ass espionage story, and a fun way to learn more about eastern martial arts and the Buddhist traditions of Tibet.
Tulku: "If I weren't the living embodiment of the Bodhisattva Avalokitesvara, the pinnacle of mercy and compassion, I would kick your lazy ass so hard you wouldn't need to catch a flight."