I simply held it in place with the thumb and middle / ring fingers of my left hand while dremeling it with my right.

If you are sanding near the spine, make sure the wheel rotates to the spine.
If you are sanding near the edge, make sure the wheel rotates towards the edge.
Failing to do so will violently jerk the dremel and possibly the blade if the wheel grabs the edge or the side of the spine.

I did not dull the edge. I never do.
By the time I am done sanding, the edge usually passes the Thumb Nail Test from all the sanding with high grit paper.