E.E. ‚Doc‘ Smith 
Skylark DuQuesne [EPUB ebook] 
Skylark Book 4

Scientists Dick Seaton and Marc Du Quesne were the deadliest enemies in the galaxy. Their feud had blazed among the stars and challenged the history of a thousand planets. But now a threat from outside the galaxy drove them into a desperate alliance as hordes of strange aliens stormed through space on a collision course with Man. Seaton and Du Quesne fought side by side to fend off the invasion – as Seaton kept constant, perilous watch for Du Quesne’s inevitable double-cross.