How to Compose for Brass Instruments: 5 Tips for Student Composers – article by Jon Gorrie
4 Tips on Orchestration and Horns – article by Bruce Hembd on how to write for horn.
Composers Who Can’t Write for Horn – article by John Ericson on how not to write for horn.
The Orchestra: A User’s Manual: The Horn – interesting site by the London Philharmonia with information on orchestration, instruments, various resources, and historical information. Includes videos, photos, and diagrams.