For some publishers the use of a template is very common place too, you simply input the details of your book’s chosen trim size, paper color, interior color, binding type, page count and (in some cases) ISBN, the template is then created and available to download ready for use.
It’s also worth considering the systems and software that you’ll be using to create your book cover, for most professionals the use of Adobe is a very popular choice (and far more accessible these days than it has ever been), they make the layout of your book cover far more precise and interchangeable between software packages, if you design artwork in Photoshop then ‘place’ it into your design within InDesign, any changes you make in PS will be updated in ID with the click of a button.
What InDesign also does very well is in enabling you to alter the design when the page count changes, if you have based the layout of your book cover upon a template then that’s great as long as your page count stays the same, but if it alters (as it normally does) then you will need to alter the width of the spine. In InDesign you are able to adjust the spine (or Gutter) very easily along with the overall dimensions of the book’s trim size, again, you can also go back to Photoshop at the same time to alter the artwork to line up with the spine too.
Book cover designers.