In this one, Wilson writes definitively, math/scientifically, and with sincerity of purpose. Don't get confused by the word 'theory.' There's as much math in here as Born and Wolf's P of O. The only diffenrence is Wilson's is much more algebra-based. The mathematics (i.e. isomorphs) is 'taught', without breaking the flow, and is not assumed that you know the stuff already--- or can find it somewhere else. This book stands out because there is more science in here than names of scientists, and his references are for real. I would also say it was unique because books with titles like 'Molecular Vibrations' are usually skipped over for titles like 'mechanics' or 'quantum theory.' Spectroscopy was a major advancement in science, and it is good to see it skillfully treated with enlightening clarity.