When Co-Author Credit is Removed

Dear Rich: Authors A, B, C, and D are researchers who together write Article X that is published in Journal Y. Authors A, B, C, v D also write a report for Foundation Z, which funded their research. The report goes unpublished except for on the Internet. Later, Authors A and B decide to coauthor a book. Authors C and D will not author anything in the book. Authors A and B have permission from Journal Y (the copyright holder of Article X) to reprint that article in the book. Authors A and B would also like to reprint or adapt parts of the report written for Foundation Z. Do Authors A and B need to obtain permission from Authors C and D to reprint or adapt sections of the report written for Foundation Z? Is it considered plagiarism that Authors C and D are coauthors of Article X but not of the reprinted version in Authors A and B's book? If so, is there a cause of action that Authors C and D can pursue against Authors A and B? Wow, that's a lot of ABC's! We’re assuming that the researchers were independent contractors, not employees of the foundation and that you all assigned your interests in the article to the journal and the foundation.
Okay so far ... If this is accurate, and the journal and foundation own their respective copyrights, you don’t need permission from Authors C and D to reprint or adapt portions. Authors C and D no longer have copyright in those works. If, however, the co-authors own the copyright in the foundation report, then the co-authors are co-owners. In that case, you can still reproduce the report without the consent of C and D, assuming you're not contractually bound otherwise, and provided that you share your book revenues with C and D based on the value of their contribution. (Yes, we know ... good luck figuring that out!)
Credits. Although copyright law does not require attribution, Authors C and D may still have a legal claim if their name is removed from the article, particularly if C and D can prove that removing their names caused injury to C and D's  professional reputation or that you had an agreement to share credit. This is important if you work in a field where credit for publications is crucial for job advancement. You can avoid the potential hassle by crediting the article and the report in the same way as they were originally published (By the way, most copyright experts agree that making a work publicly available on the Internet is a form of publication.)