A statement such as this "© 2017 X" means that the author want you to know that he is the author of this page, and that he has the copyright over the content of this page. In simple words, this means that this is some original content and that it belongs to him.
If you want to copy some of that content in your own webpage, book, or other documents, or modify it then you should do it in a way that will respect the copyright law and the license of the content chosen by the author. If the author does not provide a specific license for his webpage such as Creative Commons, then you should ask the authors about how you can reuse his content and if he agrees or not. You can send an e-mail and ask him if you can modify the content and republish it for example. Maybe he will say yes, maybe no
But beside that, you can still cite his work. For example, if you want to copy a few sentences of text from his webpage, you could use the quotation marks " " and cite his webpage as the source of the text. In this case, you typically don't need to ask the permission for that because you are citing a short part of the text.
By the way, a book can be open-source (you can see the code of the book), but "open-source" does not mean that you are free to do whatever you want with the content. The copyright still belong to the author and you need to follow the license that he choses.