Horn teachers tend to teach out of materials that they used as students with their own students up to the point that they find that they can’t cover what they need to as effectively as they would wish.
In late high school the first materials I really used in lessons were the Max Pottag, Preparatory Melodies to Solo Work for French Horn (Belwin—these etudes are by Schantl) and G. Kopprasch 60 Selected Studies for French Horn, book 1 (Carl Fischer). This was supplemented soon with Maxime-Alphonse Two Hundred New Melodic and Gradual Studies, volume 1 (Alphonse Leduc). In college a teacher the summer between my freshman and sophomore years added in Gabriel Pares, Pares Scales (Rubank), which I credit as a key to sorting out elements of my technique. The book is repetitive and could extend better into the low range but is very affordable.
I kept tracking forward through these materials generally, and at the end of my undergraduate studies another teacher added in Hermann Neuling, 30 Spezial-Etuden fur tiefes Horn (Pro Musica Verlag), materials I also particularly credit with being very effective at the time. There were excursions into other materials including a few of the Verne Reynolds 48 Etudes (G. Schirmer) and more exotic materials but actually this compact list certainly includes a good 95% of what I actually studied in lessons of this type technical and etude material until the end of my Doctoral studies, balanced of course with study of excerpts and solos starting in high school.
I started out as a teacher largely using these materials and experimented with other materials that students already owned from their prior teachers. Balance is a key element.
Over time I began to regularly request some different materials than my teachers used with me, and I developed some of my own. More on that as the series continues.