The first annotated link. Starting with Bruce Edgar's midrange horn article in Speaker Builder Magazine (cover illustration linked from http://www.alkeng.com/). A famous horn-diy-er named Volvotreter has the article scanned and posted on his website. He has a lot more on there, both horn theory articles in general as well as spectacular projects of his own. I give you a general link to his website instead of links to all individual pages. His pages are worth a visit, you navigate from there. http://www.volvotreter.de/dl-section.htm (scroll down to articles/the Edgar Midrange Horn)
In this publication from 1986, Edgar explores characteristics of midrange horns using cone drivers and arrives at strategies and guidelines for midrange horn designs. These are still the basis of his own product range, so heads up if you want to learn.
An interesting feature that Edgar refers to, is that you can tune the horn to give a different response characteristic. You can tune a horn to compliment a specific driver's response peculiarities. Given time and dedication, you can squeeze the most out of whatever potential your driver has.
Next time, a first episode in the series about Getting Started with Hornresp.