I don't have any recommendations, but if all you want is theory, then get a thermodynamics book, most of them cover heat pumps. All internal combustion engines break down into thermodynamics. You can factor power output with a typical efficiency percentage for different types of engines (piston, turbine; Otto, Carnot cycle, etc). But, i don't know how this will help you build an engine The old heat pump theory will tell you what power you theoretically could make. But get a real engine book if you plan to do much with it. Sorry i don't have any suggestions. Goodluck with your project, and never say die!