Masters of Space by E. E. ' Doc' Smith, Science Fiction, Adventure, Space Opera

Jarvis Hilton is in charge of a handpicked team of scientists -- and a navy warship on a deep space mission, dubbed Project Theta Orionis. On the outbound journey, automatic sensors drop their ship -- the "Perseus" -- into normal space for an encounter with a mysterious skeletal object. The encounter leaves them baffled, and leads them to a view an epic space battle. But strange as that is, it's nowhere near as mysterious as finding a planet covered in radioactive fuel ore -- and populated with humanoid robots who have been expecting their arrival and greet them as returning masters. . . !