I have finished The Romantic Generation by Charles Rosen. It derives from a lecture series, and probably made more sense in that context where the examples could be played and then discussed. He is a pianist, and his insights are directed toward pianists. He is familiar with large portions of this repertoire and writes in great detail about it. But if you don't have a piano, or couldn't play the examples even if you did, there is not going to be a lot for you here.
If I wrote a book about the same period, which I am not proposing to do, it would be much different. Such a book would talk about vocal repertoire, possibly the same song cycles, but very little other overlap. I would be equally biased and lopsided in my presentation. I might choose Berlioz as the leader.
Rosen places Robert Schumann at the top of his hierarchy, a choice which I wouldn't dream of arguing with. It is sad to read how Schumann went over his youthful compositions toward the end of his life and removed anything unorthodox. Times had changed away from the adventuresome romantics.
Cecilia Bartoli's Maria album is possibly a better thing to compare it to than my hypothetical book. Hers is also a lecture series with only the examples left in. Play it juxtaposed to Opera Proibita or one of her Mozart CDs, and the changes brought by romanticism will stand out in vivid detail. Principle would be the rhythmic freedom and flexibility. I find it interesting that the two things--book and album--have come into my life together.
Rosen tries to be interested in Bellini, but otherwise dismisses the entire opera genre as trash, a position we opera fans are not likely to sympathize with.
It is a forest and trees situation. There are an overwhelming number of trees (analyses of specific examples) with only an occasional hint at a forest (identification of a general stylistic trend.) If you are a pianist, this will help you understand the pieces you are playing. For others the usefulness is limited.