Hello Rabbi,

Following up on the questioner, digital copies of some of your books have been posted here on the site.
Do you permit reading them only for someone who bought physical copies of them?

Whatever appears here is certainly meant to be read.

Is there room to distinguish in a case where I already bought a physical copy of the book, and I want to use the digital copy as well (for example, so I can have the book with me everywhere, or so that I can search in it, and the like), or is it still theft nonetheless?

There is room to distinguish, and still in my opinion these are two different products and it is forbidden. For one-time use, such as looking something up, there is room to be lenient.

In my opinion, violating copyright is completely forbidden, but it still is not theft in its ordinary sense, and that has far-reaching halakhic and moral significance, since this is a prohibition whose boundaries are not absolute and clear, and therefore there is room to apply common sense and external considerations, unlike the case of outright theft, which is an absolute prohibition in every case.
The halakhic and normative difference stems from the fact that ownership can never arise from a reason; ownership is a certain legal status in which the object is in the person’s possession, and therefore the person can prevent everyone else from taking what is his. The prohibition stems from the very fact that the money or property is his, not from the fact that he will be harmed if they take it from him—just as he can prevent others from taking his legs and fingers. But the abstract creation that a person made is not being held by him or taken by him; even if we assume it has some sort of ontological status, it still is not in the person’s domain. Therefore, a person has no power to prevent others from copying that creation. Taking a physical book from a person’s house deprives him of something he has, and therefore it is equivalent to taking his fingers. But copying a work is indeed an infringement of his rights, yet it is still not taking something from his domain, and therefore there is a significant difference between them. The prohibition of theft applies in every situation and every circumstance, since a person has no permission to act within another person’s domain and boundary, and no consideration in the world can help permit him to act outside his own boundary. But copying a work created by someone else does take place within the person’s own boundary; it is not entering and invading another person’s domain. It is just that there is a good reason to refrain from doing it. Just as there is a reason not to insult or humiliate another person: a person has the legal right to insult people—it is a use of his own mouth and limbs—but morally it is not proper to do so. Yet that is not an act that a person lacks the power or right to do, something outside his domain and boundary altogether, where someone else limits and coerces and prevents him from doing it. Therefore this is a moral prohibition, not a legal one, and there is room to apply common sense and weigh each case on its own merits.