The development of Beethoven's style after 1803 indeed marks a massive expansion of the classical style into something unrecogniseable, but it is not Romanticism in the contemporary sense as was practiced by composers such as Rossini, Weber, Schubert and Voříšek (among others), and apart from a few compositions written around 1813-16 (the piano sonata Op. 90, An die ferne Geliebte and the last two cello sonatas) there is very little in Beethoven's music that even acknowledged the new Romantic trends. His late works in particular are practically reactionary, looking back past Mozart to Bach and Handel as models, while at the same time retaining the individual elements of his own style derived from Classicism. (How far this style went can be exemplified by pieces like the Grosse Fuge and the Heiliger Dankgesang, which would remain unparalleled until the 20th century.)
He is very much an adherent of the Haydn-Mozart-Beethoven trinity, but Charles Rosen's books may prove useful—The Classical Style as well as Sonata Forms (which does discuss other composers as well, though not at as great length). Also Leonard Ratner's Classic Music