I found this website the other day. It's great because it lists all the books & literature that are used in the University of Mississippi horn department. The first course, preparatory horn (100), is actually pre college level and is probably what you should start with. (Farkas, Getchell & Clarke's plus scales.)
http://www.olemiss.edu/depts/music/pdf/HornSyllabus.pdf
I found this list very interesting becuase it's basically the same books that I went through during my first year of private lessons. Valerie in Tacoma