Ace Double D-351 (1959): The Sun Smasher, bound with Starhaven.

The Sun Smasher by Edmond Hamilton. Cover by Ed Emshwiller; 110 pages. Magazine version as "Starman Come Home" in Universe September 1954.
[Here's the first-page blurb]: "It can't be true! It must be some kind of hoax!" These were the words that went spinning through Neil Banning's mind when the Greenville authorities told him that the house he had grown up in, the aunt and uncle who had raised him, had never existed. So Banning found himself in jail, charged with disturbing the peace - and maybe insanity. But when a stranger from outer space came to his cell at midnight and hailed him as the Valkar of Katuun, then Banning decided that maybe the authorities were right, maybe he was crazy. Because the only other alternative was to believe the impossible explanation of the Outworlder - that he really was the exiled ruler of a remote star-world, and the personality of Neil Banning was an elaborate fraud. It didn't really matter, though, who was right. Banning was on his way to Katuun whether he liked it or not. And as Banning - or the Valkar - he would have to save that star-world from the terror of THE SUN SMASHER… or perish with the loyal subjects he might never even have known!

(Bound with) Starhaven, by Robert Silverberg writing as "Ivar Jorgenson. Cover by Ed Valigursky; 146 pages. Magazine version as "Thunder Over Starhaven" in Science Fiction Adventures October 1957, and hardcover in 1958 from Avalon (all as by Jorgenson.)
[The first-page blurb]: After seven years of beachcombing on the pleasure planet of Mulciber, ex-engineer Johnny Mantell thought he had hit rock bottom. But when he was unjustly accused of murder, he knew there was worse to come. Johnny had to get out. And the only place for an outcast like himself was the impregnable outlaw world of Starhaven, a refuge that defied all galactic laws. Once there, Johnny's only wish was to forget the past and be left alone. But the super-science dictator of Starhaven had other plans for him. And soon Johnny found himself in the the midst of one of the most explosive struggle any world had ever known. If he failed, not only his own life would be lost, but the future of galactic civilization would be totally altered.