Harry Harrison 
Bill, the Galactic Hero: The Planet of the Robot Slaves [EPUB ebook] 

Bill would give his right arm to defend his Emperor against the alien Chingers – which is lucky seeing as he has two of them… War demands sacrifices, and if you’ve lot one left arm, have an artificial foot and a set of nifty surgically-implanted tusks, it’s a small price to pay for the privilege of being a hero. And Bill knows all about heroism – as part of a motley crew his new task is to track down the source of Chinger-controlled metal dragons that are making mincemeat out of humans…