To grasp "what architectural fashion is really arguing about"
Follow the "ornament vs. structure" main line: the classical and Gothic took ornament to the extreme, but with the industrial age Sullivan, Le Corbusier, and Mies raised the modernist revolution, crying "form follows function" and "less is more," demanding that a building honestly express its structure and throw away all excess. Yet in the contemporary, Gehry and Zaha made architecture twist and flow again through the computer. You will find that the history of architecture is largely a swinging back and forth between "ornament or not, and who decides the form."