It's probably not wrong with a layman using an e-reader to follow the Mass. The more I think about it, however, the more I wonder if it might be distracting to others.
I would think an e-reader on the altar would be wrong; it would certainly look silly. I think it would also violate the spirit and the letter of the rubrics.
I have seen priests use an iPad or something similar when delivering the sermon but never on the altar.