Laurence Yep 
Shadow Lord [EPUB ebook] 

Angira is a primitive, violent planet — and young Prince Vikram returns from Earth filled with new ideas. When Sulu and Spock accompany Vikram home, they walk into a bloodbath: reactionary forces, afraid of any modernization, have seized Vikram’s rightful throne. Suddenly the men from the Enterprise are on an underground journey with a Prince who is coming of age. The future of Angira is at stake, and each man’s survival depends on his skill — and daring — with a sword!