Piano sonatas (Beethoven)
Ludwig van Beethoven wrote his 32 piano sonatas between 1795 and 1822. Although originally not intended to be a meaningful whole, as a set they compose one of the most important collections of works in the history of music. Hans von Bülow called them "The New Testament" of the piano literature (Johann Sebastian Bach's "The Well-Tempered Clavier" being "The Old Testament").
Variations on an original theme in E-flat major (Beethoven)
The Variations in E-flat major piano trio, Op. 44, by Ludwig van Beethoven, is a series of fourteen variations on a theme, written for piano, violin and cello. Although this may be one of Beethoven's early works (written at around age 22) it was assigned its opus number when it was published by Hoffmeister in Leipzig, more than a decade after Beethoven began writing it.