Obviously any original work you have written yourself is free from copyright fees; but be careful as to how you incorporate references from others' writing, and other people's ideas.
A small passage from a book or paper may be deemed a reference but it should be acknowledged. If it is significant, it still may need to be cleared.
If you want to use any more than a reference you are legally required to get written permission or to licence.
After a certain number of years have elapsed, copyright material moves into the public domain. The timing of this varies from country to country.
Last Updated: August 08, 2007