This book displays and talks about the work of Blake, Constable, David, Degas, Delacroix, Fuseli, Géricault, Goya, Ingres, Millet, Piranesi, Rodin, and Turner. On the one hand, about half of the space in the book are paintings and drawings, and you could enjoy flipping through the book, seeing images, and reading some commentary on them. On the other hand, you can read the book for the text with the images as examples that make tangible what Clark has to say about the styles of the artists.
Clark does not write abstractly about classicism and Romanticism, rather he presents representative works of the above artists as case studies and says concrete things about them. He gives in passing a few general descriptions of classicism: when talking about Géricault he writes "The godlike calm, the stasis, the restraint of academic classicism were not in his nature", and at the end of the book he says about Romanticism, "To use the body as a means of expressing the anguish of the human soul is no longer a possible enterprise; we do not know how to represent the body and do not believe in the existence of the soul."
In classic art there are timeless scenes where the participants have settled minds. Clark writes that "The classicists believed, in Winckelmann's words, that 'art should aim at noble simplicity and calm grandeur'; the romantics said that art should excite the emotions, and in particular the emotion of fear, which was the source of the sublime." Winckelmann's phrase "noble simplicity and calm grandeur" ["eine edle Einfalt, und eine stille Größe"] is from his Gendaken. It appears in the following context in Fuseli's translation: "The last and most eminent characteristic of the Greek works is a noble simplicity and sedate grandeur in gesture and expression. As the bottom of the sea lies peaceful beneath a foaming surface, a great soul lies sedate beneath the strife of passions in Greek figures." ["Das allgemeine vorzügliche Kennzeichen der griechischen Meisterstücke ist endlich eine edle Einfalt, und eine stille Größe, sowohl in der Stellung als im Ausdrucke. So wie die Tiefe des Meers allezeit ruhig bleibt, die Oberfläche mag noch so wüten, ebenso zeiget der Ausdruck in den Figuren der Griechen bei allen Leidenschaften eine große und gesetzte Seele."]
Clark makes a perceptive comment about Ingres getting ideas for his paintings from Flaxman that I think applies to other fields: "It is a mistake to say (as is often said) that he [Ingres] had no power of imagination, but the central formal ideas which lay almost out of reach at the back of his mind were so obsessive that they restricted free invention. The exact reverse was true of Flaxman; and precisely because he was devoid of a dominating central idea he could pour out inventions. The fact that they were dliuted and descriptive did not make them less valuable to those greater artists who were in need of a precipitant."