Wow, I'm glad to see that the book is selling so well. So does that mean the entire first printing has sold out, or just the ones at S&S's online store?
It means that warehouse inventory was quickly and completely depleted, and that a rush second printing is necessary to meet the exceptionally high reorder activity.
Now when can we expect the eBook?
I can't answer that with any greater specificity than "soon." It isn't an aspect of the publishing process over which I have any influence.