GRAPHICNOVEL001 is a generic adhesive-bound graphic novel. Graphic novels can range from a few dozen pages to a few hundred, this PSD Mockup represents the thickness of about 300 pages and used the WATCHMEN graphic novel as the product template. Graphic novels, like comic books, have a standard-ish product dimension. While this particular product measures 168mm x 258mm (6.6in x 10.1in) others can be shorter or wider by up to 5mm (3/8), all the rendered samples in this post for example used content that came from graphic novels whose dimensions varied very slightly.
This is a product shot, not a manufacuring spec, don’t worry about squeazing your work to fit. The final product is rendered standing up at a roughly 45° angle facing forward, the view is from straight on at the center of the graphic novel. In this shot, only the spine and cover designs for your book are visible — you can render the back of the book simply by placing your artwork into the template as usual and then flipping the artwork in the SPINE area horizontally and in the COVER area horizontally.
Essentially the template remains the same but the artwork is reversed. After rendering STEP 2 the graphics on the product will be reversed, simply flip the entire canvas horizontally and your design will now appear properly and you will have created a render of the back of your book.