Pierre de Pré wrote these studies specifically to help hornists practice the low range of their instrument. Playing with full control in the low register requires subtlety and focus, and these etudes are exactly what will help you get there.
The variety of styles, articulations, dynamics, and technicality will help every player develop their low range abilities. The music routinely going between bass and treble clef to help you practice clef transitions.
The first two pages are available to try for free to the left, then you can click above to get an immediate PDF download of the whole book.