The Well-Tempered Clavier
Both collections have 24 short preludes and 24 fugues to go along with it. All of them contrast with each other: some of them are easy to learn, while others are hard to follow (because there are many different melodies going on at the same time, which is called counterpoint). What is special about them is that each prelude and fugue is written in a different major or minor key. They appear in the book so that they go up by semitones. That is, the first prelude and fugue is in C major, the second is in C minor, the third in C-sharp major (or D-flat major), the fourth in C-sharp minor, and so on, until the last two preludes and fugues are in B major and B minor, in that order.