All psds belong to their rightful owner, I don't own any of these.

A PSD, which stands for Photoshop Document, is the default format that Photoshop uses for saving data. PSD is a proprietary file that allows the user to work with the images' individual layers even after the file has been saved. Adding a psd can add different tones and colours to your cover which can make or break a cover.

This is a PSD created by nightgate and as you can see ➡️ shows the original picture while ⬅️ shows the after effect of the psd once it is added. As you can see, it has added layers of tone and depth to the image making it look better.

There are examples where it doesn't work like so:

This has no psd and it looks alright, there is nothing really that should be changed. This is with a PSD at it's lowest:

As you can see it has darkened the cover so much that it is hard to see the book number and title (depending on your sight).

With every PSD there is always a theme that works with it like so:

If you are wanting a dark cover go for something dark that will make the psd work; everything is about EXPERIMENTING.

Experiment with as many PSDs as you can find and test them to see which cover fits a dark cover or a light cover, that's how I found out things.