That's what I'm going by, although I wonder if digital piracy constitutes theft for the reason Mithrandylan says. Can there be such a thing as non-physical property? If no, you could still argue that a digital book is physical.
If digital piracy is a sin, as I would assume, what would that mean for the Library subforum on this site?
Not a whole lot since the majority of the books posted there are outside of their copyright in the US.
Also, it may be theft, but would at worst be a venial sin as the matter here isn't grave. You're making a copy of a copy. The owner still has the object. Especially if it's some book that the author or publisher doesn't make readily available (out of print, publisher defunct) while technically still in copyright, it doesn't prevent them from receiving funds for the work since there's no one to receive them.
Do you have the same moral dilemma for buying a used book? The author and publisher are not receiving funds for your purchase of it, just the from the person who bought it new.
Now, if you're posting links to movies or brand new media just to circuмvent paying for it, then yes, that could be argued as a grave sin (I thank you Lad for clarifying that term for me now) because the intent is to defraud someone of revenue.