Is it Legal or not?

if you don't sell the book and use it only privately that's ok. If you want to sell the book you have to respect copyright.

As long as you respect the copyright, give your sources properly, ask the original author to use the content on your book, and be sure you are on the safe side, it should be fine. But you may need the permission of the author.

I think it depends on what you are copying. Like if there's a site about how to tie your shoelaces, and you write a book about it. Would you be offending the law? Of course not! If knowledge is copyrighted, then this world is upside down... But if you find something very specific on the web, such as how to make some sort of cake, and you directly copy the instructions, you'll be offending the law.

There is just a lot with internet. It truly is helpful to have all these information in front of you. Making a good use of these information would not hurt I believe. I would think that people who spent much of their time doing research just to have this things available to us would be happy that their work helps us. However, claiming their work is another thing. I think that it would be alright to use their work as long as we recognize them and not let people believe that it was you who made that. Studies and researches are of a lot of work. It would not be so nice to exploit them in any way. Have their names published also friend. I think that in this way, things will work out fine.

Copying the exact content of a writer and making it yours is plagarism. hope you won't be penalized anyway. However, you can use the idea gotten from the write up to make up yours.

It will not be ilegal to make a book based on the information you found on the internet, with your own prespective. But it will be ilegal if it's copied in the same exact form.