Registration is for each work individually; thus, example, if you have a 7 book series, each book would need to be registered individually.Remember: copyright inures in the act of creation. That means that the creator of an original work--like, for example, an author--automatically has the copyright in anything (s)he creates. Copyright registration is not, strictly speaking, necessary. What registration does is to provide notice to everyone that you are claiming copyright--so there is no confusion as to whether the work is copyrighted, or who the copyright owner is--and also allows for a larger recovery (more compensation) in the event of infringement.