Personally I often download books from these kind of websites, as I don't use DRMs. What I do also is buy a physical book and then pay the service to get a digital copy by giving the book to a business which converts it to a pdf for me. Typically I do it for books on Amazon. With DRMs is it moral to lend a book to your wife for example? I believe so but Amazon doesn't since with DRMs I can't even use it myself without installing proprietary software.

So my solution: buy the physical book then share it with friends and my wife (totally legit for physical books but this is called piracy for digital versions) OR download it from a website and then give some money to the author (most often this option is not available so I give nothing) OR buy the book from a website which makes the book available without DRMs so I can read it without proprietary software and share it.

In the past at school we had to buy books, then the following year we had to sell it to the next students. Incredible waste of time and money. This is not possible with digital books with DRMs, and to be honest I would be in favor of the school buying the digital version and sharing it with the class. This is actually what I do currently, I use digital copies of textbooks and never even had the physical copy in my hands.

For technical book I see sometimes a choice of how much we want to give for the author, I like it as there is a reasonable minimum. Technical books by the way can be quite expensive, the price can easily go up to $100-$200.

Morally I guess Intellectual Property brings similar problems we have with source code. Is it morally ok to share the source code of Bitcoin for example or any other program, whatever its license, without giving money to the author? I think so but the business model which goes with it is/was disruptive for businesses which distribute proprietary programs.