Short note before anything else: I'm not a lawyer, this isn't legal advise, and if you do or don't do anything based on what I'm explaining here, I'm not responsible for it ;)
According to copyright law (called "Urheberrecht" in German), just about any and all creative material is automatically copyrighted. For anything that is above the so called threshold of originality, the creator ("Urheber") retains certain rights. Among those rights is the right to create copies of the work.
I think we can safely assume that the vast majority of books easily get above the threshold of originality. So you can safely assume that books are generally fully copyrighted.
Creators can basically decide what to allow or not allow others do to with their works. For example, a creator could allow others to create copies of their works and distribute those copies. But they don't have to allow it. In many cases there's a notice what the creator wants to allow, sometimes called a "license".
In German copyright law, there are a handful of cases where you can do something with somebody else's work even without explicit permission, for example you may quote short(!) excerpts of a work in your own work. But those don't reach very far.
The books you're referring to have been published by a professional publisher not many years ago. So you can safely assume that they want to be paid for their work, because that's how they make a living. If you find copies of those books that aren't explicitly authorized by the publisher, those copies are probably illegal.
As a side note: The very basic principles I've described here not only apply for German copyright law, but for the laws of most countries. That has been layed down in the Berne Convention, to which nearly 180 countries are party.