Histories of Instruments and Instrument Makers
The Reynolds company offers an online history of some of their horns, including the Pottag model, the Chambers model (depicted, right), the Contempora double and single, the Emperor single, the Roth single, the Reynolds piston valve horn (single in F with crooks in Eb and D), and the Regent.
Carl Geyer - Chicago 4/9/63 - a Reminiscence by Thomas Bacon
The Double Horn and Its Invention in 1897 - article by John Ericson
King: The H. N. White Story - The story of King instruments
Holton Memoirs - by horn maker Wes Hatch: Part I (Introduction), Part II, Part III
The Impact of Technology on Musical Instruments - article by Edmund Bowles. See the section on "The Venerable Valve"
A Brief History of the Conn Company
Margaret Downie Banks, curator of the Musical Instruments National Museum in Vermillion, South Dakota, has compiled an extensive collection of information about the Conn Instrument Company (now owned by United Musical Instruments). Also by MDB: Publications about the Conn Company
List of Horn Manufacturers - this list comes from the Wikipedia; not all manufacturers listed here are still in existence.
Publications about U.S. Musical Instruments and Their Products, Publications about Brass and Woodwind Instruments - by Margaret Downie Banks
History of the G. Leblanc Corporation - parent company for many years of Holton
Making Rotary Valves for F.E. Olds Company - memoir by Bernie Marston
The Vienna Horn - (pdf) - translation of the thesis of Thomas Jöbstl, then principal horn of the Vienna Volksoper (2001), now of Die Wiener Philharmoniker
Cottrell, David. "Walter A. Lawson, hornmaker." DMA diss. University of Oklahoma, 1988. ProQuest ID: 754048661