I checked the index to James Robertson's biography but could not find the documentation.
Ah, a Google search Stonewall Jackson and Slavery reveals the following:
Stonewall Jackson, Champion of Black Literacy by R.G. Williams, Jr.
"Mention the legendary Confederate General Stonewall Jackson to most people and the image that immediately comes to mind is one of a fearless, hard-fighting Southerner known for his eccentricities, who some say fought for slavery. But Thomas Jonathan Jackson was a much more complicated man.
Indeed, a careful study of his life would lead one to believe that General Jackson might even be described as a civil-rights leader. Yes, that’s right, a civil-rights leader. In the nineteenth century, prior to the War of Federal Aggression, Virginia law prohibited whites from teaching blacks to read and write. Though Stonewall Jackson was known as an upstanding and law-abiding citizen in Lexington, he routinely broke this law every Sunday."
http://www.lewrockwell.com/orig2/williams1.html