If you are preparing your own cover file PDF you will need to know how thick the spine will be. We can tell you this when you know how many pages are in your book. We can confirm the spine thickness to you and give you a template for your cover design, but there are a few other factors to take into account.
If you are designing your book for the UK or American market the convention is that when the book is upright on a bookshelf the base line of the spine lettering is on the left. If you are designing your book for the European market the convention is that when the book is upright on a bookshelf the base line of the spine lettering is on the right.
When paperback book covers are attached to the book block during manufacturing (that’s the block of paper that comprises the inside pages), it is not uncommon for a small amount of slippage to occur. This is more likely to happen with print on demand books and short run printing than with long run printing. What is the effect of this? Well, suppose you had a solid red spine and a white front cover. You could end up with a line of red visible on the front of the book so I would try to avoid really sharp contrast between the spine design and the front cover with print on demand books. Similarly any text on the spine or critical graphical element like a publisher’s logo should be not less than 10mm from the top or bottom trimmed edge of the spine nor less than 1.5mm from the front or back cover. For some books (typically those with less than 80 pages) this means you won’t be able to put any text on the spine.