The Literature section of the Forum of the web site of the International Horn Society also includes questions and comments about horn etudes.
Kopprasch Studies - discussion on Hornplayer.net
Index to Horn Etudes - article by Jeffrey Agrell, categorizing specific etudes according to technique
The Personal Etude - article by Jeffrey Agrell on how and why to compose your own etudes
Twenty Difficult Etudes for the Horn's Middle Register -by Daniel Grabois of the Meridian Arts Ensemble