The word free in the English language does not distinguish between free of charge and freedom.
Free of charge means that you don’t have to pay for the book you received.
Freedom denotes that you may do as you like with the book you received.
This distinction is immaterial if you just want to read a book privately, but it becomes of utmost importance if you want to work with the book:
If the book you got is just free of charge but protected under U.S. copyright law, you may do none of the above things. You may not even make a copy of the book and give it to your best friend. But if the book you got is free as in freedom you may do anything you like with that book. Clearly free as in freedom beats free of charge.
Fortunately almost all Project Gutenberg eBooks are free of charge and free as in freedom for readers within the United States (if you are not in the United States, you will need to determine what copyright law protects where you are located).
Here are some real world examples of what people did with Project Gutenberg eBooks.
A few Project Gutenberg eBooks are protected by U.S. copyright law. You can tell by reading the license inside the book. You may download our copyrighted books and give copies away, but might be limited in commercial uses and derivative works.
Copyright For the most part, copyright protection under U.S. law on the materials distributed by Project Gutenberg has expired. (They may still be copyrighted in other countries). So anybody located in the United States may make verbatim or non-verbatim copies of those works.