If forced, by the proverbial gun to the head, to name the highlight of my two visits to the Prado, I would say Goya's "Black Paintings." By no means do I have an extensive background in the fine arts, either through second-hand study or first-hand experience at museums, but the Black Paintings is the most awesome (in the old sense of the word) set of art works I have ever encountered, and "Saturn" and "The Dog" are among the most powerful single images I have seen. To think that they were painted on the walls of Goya's private residence with, apparently, no intent that they be displayed publicly is mind-boggling.
Alas, this book, apparently the only book in English devoted solely to the Black Paintings currently available, is a major disappointment. The quality of the reproductions is only average (in particular, the colors seem to be somewhat washed out), and the precious little accompanying commentary is mediocre at best and appears to have been given a translation of comparably middling quality.
The only reason I give this as much as three stars is because the subject, the Black Paintings themselves, is such a monumental achievement of art and human genius.