What is the ruling on downloading non-free books from websites using hacking or sharing programs?

The books available on websites, some of which can be downloaded and some are for browsing only. There is no issue with books that are permitted to be downloaded. As for books that are prohibited from being downloaded, the general rule is prohibition, because their owners have rights that would be lost with free downloading. The method mentioned in the question (Peer-to-Peer) is what hackers use to seize copyrighted material. Fatwas have been issued prohibiting the seizure of copyrighted materials without the owners' consent.