35 or 40? That's all?
That's why I'm not impressed by most of the historiography regarding black confederates. Were there servants who went with their masters, slaves hired out by their masters to dig ditches, tend horses, and cook? Of course there were. But it's a far cry from that to making some sort of argument that the war wasn't about slavery, which seems to be the agenda of these works.
Oh, and I've read plenty of the pro-southern stuff, from Jefferson Davis to Tommy DiLorenzo. Pretending that these works are unbiased while other works are hopelessly biased is simply absurd.
Oh by the way, there are also some great books out there concerning Indians, Hispanics, Jews and women who served in the Confederate army. You can read those too if you have the time. :-)