This is a set of two books of exercises for French horn. They are part of the standard repertoire for advancing horn players.
Book One: A necessity for all horn players. This is a great book to start off your intermediate studies.
Book Two: A necessity for all horn players. More advanced than book 1.
From french-horn.net:
Bernhard Edward Mueller was born on June 2, 1842 in Altenburg / Thuringia. He was the son of Hooper Wilhelm Hermann Muller. He was a the second horn player of the Gewandhaus Orchestra in Leipzig from 1876 to 1920. He may be the same Müller who played in the court orchestra in Schwerin, which from 1872 - 1874.
Müller has become known primarily for his Etudes for Horn, but he also composed numerous chamber music for Horn quartette, Emperor Quartets for cornets, bass trumpet (or alto horn), bass flugelhorn (or tenor horn), a Serenade for Horn, Flute and Piano and other chamber works.
You can hear Bernhard Eduard Muller on an old shellac record, released at the Spring Fair in 1904.