To Outrun Doomsday

The planet Kerim must have been Utopia - once. All its inhabitants had to do when they wanted something was to pray out loud for it - and what they wanted would materialise before their eyes. But by the time Jack Waley crashed on it, its best days had long been gone - and its future was strictly limited. Which was typical Jack Waley luck. He had bungled and blundered his way across the space lanes, messing up everything he tried and being castaway on Kerim looked like the end of the line.
For Kerim's people were now bands of confused savages and its cities crumbling ruins. And this time Waley knew that he'd have to change a whole world's luck if he wanted to save his own neck one more time.