> the modifications can be attributed to another person, who might have no relation with the original author.

Not really.

GNU requires that if you make changes, you still distribute the source code (it is about software only) with your changes, and the previous changes, and that the results also has a GNU license. You keep the copyright to your new code, but the original code, and the changes you made to it are copyrighted by the original owner.

A good example of when this happened is when the popular web CMS software Joomla split off from Mambo a few years ago.

Even though the progam's data formats are not very compatible any more, the underlying source code is still derived from Mambo and always will be, so only the new code added by the Joomla team is owned by them.

CC does not necessarily allow free duplication, and it applies to any intellectual property (but I have never seen it applied to source code). The Creative Commons web site explains the variations, that run from pretty much free to copy but not to change, to free to change as long as you attribute the actual source. Again, the underlying copyright remains with the original author.

Both of them allow free duplication, right? Then what is the difference between them? Also, which license allows the work to be identified as the intellectual property of the author, but still keeps the book free for publication, copying etc.? So far as I know, GNU Public License allows the original work to be modified and the modifications can be attributed to another person, who might have no relation with the original author.