What is the ruling on downloading religious books from the internet and reading them for personal benefit, especially if they are marked "copyright reserved" or are available for free while being sold on other sites, given the inability to ascertain the permission of their owners?

It is permissible to download and read books published online unless one knows that their owners have prohibited it. However, it is not permissible to download or copy copyrighted books except with the permission of their owners. As for the books of Sheikh Ibn Uthaymeen, it is permissible to benefit from them if their download and copying are authorized, and purchasing them from his website is preferable if one is able to do so. Some scholars permit downloading and copying books for personal, non-commercial use.