Carl Czerny

Czerny was Beethoven's pupil and Liszt's teacher, and he wrote around a thousand works of which the studies are what survive in use. Nearly every pianist meets him whether they intend to or not.

The sets are graded and knowing which is which saves money: Op. 599 is the beginner's book, Op. 849 and Op. 299 (The School of Velocity) are intermediate, and Op. 740 is advanced. Buying the wrong opus number is the commonest mistake in the whole teaching repertoire, because the titles are similar and the difficulty gap between them is large.

All of it is out of copyright and free scans are everywhere. What a modern edition adds is fingering and a sane selection — the complete sets run to hundreds of exercises, and no one works through all of them.