They are separate works for copyright registration purposes, because the book was published separate from the DVD years before. They both now must be registered separately.
They are being sold together now, however, so, for copyright infringement purposes, the infringement of the package (book with DVD) can lead to a single copyright infringement lawsuit.
The link I provided earlier, to the Copyright Office’s Circular 34, explains nicely what works can be registered as a “unit” (on a single application) and what cannot.
I should also note that registration of a work is not necessary for the work to have copyright protection. Copyrights are automatic once a work is fixed in tangible form (e.g., words on paper, paint on canvas, music on recorder).
Timely registration of the copyright provides the copyright holder with additional protection, however. Not only does registration provide the copyright holder with the presumption of ownership, with timely registration the copyrighted work becomes eligible for statutory damages when the copyrighted work is infringed - $750 to $30,000/infringed work, or up to $150,000/work for especially egregious infringement.
Being eligible for statutory damages if your work is infringed is a good reason to register your copyrighted work with the U.S. Copyright Office prior to publication of your work, especially since the cost of registration is so low compared to the protection it offers.