What is the ruling on downloading electronic textbooks from the internet for personal use and to further one's knowledge, given that they are uploaded without the permission of their authors who do not allow their books to be photocopied?

It is not permissible to download copyrighted books without the permission of their owners. As for non-copyrighted books, there is no harm in downloading them. Some scholars have permitted the downloading and copying of copyrighted books for those who need them and cannot afford to buy them, provided that it is for personal use only and not for earning money or engaging in trade.