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").
Sonata
It is difficult to overstate the importance of Beethoven's output of sonatas: 32 piano sonatas, plus sonatas for cello and piano or violin and piano, forming a large body of music that would over time increasingly be thought essential for any serious instrumentalist to master.