In my opinion Longyear gives Chopin short shrift (7 pages out of 367), and Chopin seems oddly situated in the chapter on Italian and French Romanticism (yes, he did most of his composing in France, but still...). I suspect Longyear thinks Chopin is a lesser composer because he wrote primarily for piano.
That aside, this was a surprisingly interesting and pleasurable read, encyclopedic for its relatively small size. It was like a survey course on 19th century Romantic composers. It's too bad it didn't come with a set of CDs so we could listen to all the musical examples cited (there are hundreds). It takes awhile to track them all down on the Youtube. (I recommend it - an education in itself. The listening, not the tracking down.)
Longyear works both chronologically and geographically, beginning with late Beethoven and ending with Franck, Verdi, Mahler, Wolf, and Strauss. He adds chapters on the styles and forms of Romanticism, and concludes with a discussion of how music was performed and heard in the 19th century.
There's a lot of technical material here. If you're not entirely familiar with music theory, you can skim these parts. (Sample: "Often when Brahms goes to the dominant side of the circle of fifths he returns via the subdominant to the tonic. The minor forms of the subdominant and the tonic are his favorite pivots for modulation, and many of his diminished-seventh chords consist of the third of the minor subdominant...") Each chapter has a nice and reasonably lengthy bibliography; the Third Edition (as far as I know this is the most recent one) dates to 1988, so presumably these bibliographies are a bit outdated now.
Some interesting excerpts, to give you an idea of Longyear's opinions and writing style:
Despite Beethoven's intense personal sincerity in this work and the sublimity of its outer movements...it is difficult to avoid considering the Missa Solemnis as one of the greatest failures in the history of music. For the work is uneven, even patchy in places, and the overlong conclusions of the Gloria and Credo, influenced by the choral writing of Handel, tend to stupefy rather than edify.
...Weber's first movements have magnificent openings, but the lyrical themes are not suitable for development, the transitions sag, the second themes are not as strong as the first themes...
In recent years the symphonies [of Schumann] have seldom been performed, and the Cello Concerto is neglected because its contrasts are too subtle, its poetic atmosphere too unrelieved, and its virtuosity too unevident, though cellists know how difficult it is.
Though Brahms was the most contrapuntal composer of the century, he was the least ostentatious about it and in this respect is surpassed only by Mozart, for even in Beethoven's music one often senses that the composer is deliberately calling attention to his use of a "learned" device.
The information is great, but the writing is atrocious. Longyear appears to have a thing for semicolons, colons, and hyphens...as they appear in virtually every sentence. These convolute his thoughts most of the time and verge on the point of the writing reading more like a series of lists rather than prose.
This is a good introductory summary of Romantic music. With a concentration by country and an emphasis on orchestral music this seems a reasonable place to start your search for information about romantic music. For more depth I would recommend Dahlhaus.