A good place to start to search articles about the horn is the Horn Call Index containing all the articles published since 1970. The Horn Call Magazine is available for download to all International Horn Society Members, JOIN NOW!. In adition to the magazine articles, there are a few online articles at the IHS website. The International Horn Society also has a E-Newsletter that you can subscribe for free.
You can also check here some Thesis & Dissertations and Books about the Horn.