One of the first important texts about orchestra conducting is this one, written by Wagner in 1869, with the title of "Über dirigieren". I prepared a Web page with its English translation: On Conducting.
I have also found an spanish translation of this book, who did someone called Julio Gómez, and was published in Madrid in 1925.
Here is a very interesting text by my friend and orchestra conductor Jacques Bodmer, who as he was participating in an Internet forum on orchestra conducting - from the idioms on the text I deduct it was mainly a latinamerican forum - he summarized in a short text for laymen, the essential part of the orchestra conducting. This led to quote some texts by Wilhelm Furtwängler. Click to read the texts by Jacques and Furtwängler.
An article-interview with Sergiu Celibidache which I collected some time ago, I don't remember from which magazine, which describes his musical thinking and his vision of the conducting job.
Here there are some notes by Arnold Schoenberg on the
metronomical indications by Beethoven.
Felix Weingartner wrote a book "On the performance of Beethoven's Symphonies". I include here a chapter with his musical analysis of the First Symphony by Beethoven. This book was written in 1905 and this is noticeable in the arguments he uses, but everything he says makes sense, even if we don't agree in everything.
An article with thoughts regarding schools of conducting, and about the orquestral conducting technique by Ylia Musin (and his disciple Ennio Nicotra).
This is a humorous text that I found on the magazine "The double reed", and therefore it could be the ironic viewpoint of a musician in the orchestra. Anyway, here is What it takes to conduct an orchestra.