Only add pages that you are sure are allowed under the terms of the GFDL.
Copyright is a kind of law that says we can't make people think that we made something that was really made by another person.
Copyright law says that we cannot pretend that we made something that was really made by another person. Copyright is used for books, movies, songs, and Web pages, including pages on Wikipedias and Main Page (like this one).
Laws about copyright were created so that people who write books or songs or make Web pages or movies could get money for their work in a fair way. If I write a good book, I want people to buy my book and give me money for it. I don't want someone else to sell my book and take the money from selling it. I also want people to know that I was the first person who could write a book like that one; if the book is good, it means I am good at writing and people can hire me to write things for them.
The rule of all the Wikibooks, including this one, is that nothing in the Wikipedia can be copyrighted by someone else unless we have permission to use it. This means that we are only allowed to copy things from books or from other Web pages into the Wikipedia if the writer has said that anyone can copy these things and make changes to them.
There is an exception to the rules of copyright called Fair Use. This says that we can copy a very short amount of something -- maybe one or two sentences, or a picture. But because people could get very angry at the Wikipedias if we copy the wrong things, it's better not to copy anything onto the Wikipedias unless we are sure it's allowed.
Different countries have different copyright laws. Most of the differences deal with whether the government's work is copyrighted, how much longer copyright lasts after the author dies, and what is fair use. Wikipedia needs to follow the copyright law of the United States, because its words and pictures are stored on computers in the United States.
Can you move things from other Wikibooks and Wikipedias to here?
Yes, you can do this. But do you have to say you did this? There is no easy answer. Under GFDL, you really should, but when a person writes for Wikibooks, they know that their work can be changed because the edit screen tells them their writing will be edited at will.
This means that copying between pages (or projects) is ok. The changes you make are more than just making the words correct, so copyright is not kept. This makes you a co-author.
You can show respect to other people by saying they did this work. You can do this by adding things in your edit summary. See Simple English Wikipedia for more information.