I'm glad I read this book. Like most people (I guess), I've watched hundreds of films, and yet, I remain appallingly ignorant about movies.
This book explores the idea of continuity of genres, techniques, styles in movies going back to the early 1900's. Bordwell offers examples of how many of the features that movie viewers would consider novelties were present very early in cinema history; that the history of cinema is marked by this continuity, particularly how the feeling of belatedness, encourages newer generations of directors to respond, adapt, improve, change, deliberately break with, etc., with what their predecessors had done (thus maintaining this tradition). This topic (which is the central topic) was very revelatory to me.
Another idea that stuck me was the fallacy of "artistic" loss in cinema compared with previous epochs. The author makes a convincing case that spectacle is not antithetical to narrative or plot, or character in movies and that, in fact, even action movies, even hero action movies, retain the elements of classic cinema and the triumphs of this continuity.