Gordon R Dickson 
Earth Lords [EPUB ebook] 

A hidden labyrinth beneath the Canadian wilderness where dwarfish Lords and Ladies ride humans like horses – and plot the final downfall of mankind. Bart Dybig is a "Steed", but one gifted with mental and physical abilities unsuspected by those who have enslaved him. Soon, he vows, he will surprise the Lords and escape to the world above – if there’s a world to go back to. For the Earth Lords are building a doomsday device of unimaginable power to completely destroy mankind. Only Bart and his strange heritage can stop them.