Two full-length and unaltered 1960's classic space opera tales reissued in one volume. Dated, but coming from a good writer (and Bova is) that doesn't seem to matter very much.
From back cover: STAR WATCHMAN---When the human race reached into space, it discovered that it had been preceded by its own ancestors--who had been ruthlessly smashed back to the stone age by a powerful alien race known only as 'the Others.' Modern mankind has been compelled to build an interstellar empire, and the officers of the Star Watch must knit together the worlds of the galaxy into a federation capable of fighting their ancient enemy.
THE DUELING MACHINE---Dueling as a means of settling disputes has been revived by the invention of the duelling machine, which allows two adversaries to have at each other in the imaginary world of their choosing, with no danger to either other than humiliation and the loss of the point in dispute--until the Kerak Worlds found a way to kill with the machine. Unless a young Star Watchman can solve the mystery, the warlike Kerak Worlds will gobble up the planets of the Acquataine Cluster, murdering its leaders one at a time, and then be ready for the Terrans themselves.