The question has two parts: How to design a book cover? How to obtain the final format as PDF/X? The PDF/X-1a or PDF/X-3 formats are often, not always, required by publishers.
Designing the cover, in terms of layout and content, can be done with the "book-cover" package, as mentioned above. You should also consider using a graphics program. GIMP works. Note that there are not many good reasons to use LaTeX for book cover design. A graphics program is better.
An ordinary PDF can be converted to PDF/X using Adobe InDesign or Acrobat Pro. These programs have sophisticated methods for altering a background image so that it print well, with overlying text. LaTeX, not having much graphics capability, cannot do that (and probably never will). GraphicsMagick can do that.
Also, the PDF/X may be required to comply with an ink limit. For example, the most prominent print-on-demand services in the U.S.A. impose a 240% ink limit for CMYK artwork.
EDIT: In my initial reply, I referred to a GitHub project. That project has now been discontinued.