Hi, Thanks for the question. It's a bit tricky to explain - basically I've got a few extra "scuff" layers at the very to of my layer stack in Photoshop:
1. A very yellow / orange aged-paper layer set to 50% Multiply.
2. A masked Brightness / Contrast layer which picks out folds and scratches.
This can vary if it's a predominately dark or light cover. If it's quite a dark cover, I tend not to bother with the yellow layer, just the scratches.
I've been meaning to do a template / tutorial on how to do one of these covers for some time now. It's far easier to explain visually than with words, I think. I'll compile that soon and post it when I can. Thanks for reminding me!