How to Create a Box Set Cover
I have twelve medieval novels and I offer boxed sets by theme. That way someone who only wants the books that involve chaste heroines, or wants to read about the heroines who have prevailed over harsh challenges, can read those stories without having to hunt them down manually.
Here's step by step instructions on how I created one of these, back with my original covers.
First, the final product. We are trying to create an image which accurately reflects to the user what they're going to get. In this case, it is four full length novels.

So, step 1. The top cover has to show that there are four books. In my medieval novels I had an emblem on each individual cover, like this:




I put each emblem on the box set cover to show these were the stories included in this set. If your book covers have integrated text and images, you might want your overall box set cover to have small versions of each full cover.
You want your original cover to be 2813 x 4500 - the Kindle proportions. Create a cover which has the box set title and features all four (or however many) sub-books in it.

OK, great! Step 1 is done. Remember, this is going to be seen at tiny thumbnail size when people are buying it, so those images have to be clear. That's part of why I love my emblems. They stay clear even as they shrink.
On to the box set creation. Make a brand new, blank image which is also 2813 x 4500. That's a good Kindle size.
Make a version of your cover which is only 2250 pixels high. Cut and paste that into your blank document. Put it against the left side, leaving a few pixels of margin. Center it top to bottom. It doesn't have to be exact.
Select the bottom left corner of your graphic. Now click the "SKEW" button in the bottom area of PhotoShop. This lets you use that anchor point to skew your image. Drag the bottom left corner up. Then drop the top left corner down. Create whatever feel of perspective you think works best for your book.

Great! Now to add in the spines of the book. On the right side, make a black rectangle which is 2250 pixels high and fits from a few pixels right of your cover to a few pixels away from the right edge. Just as before, click the far corner of it, use skew, and skew the right top and right bottom to create the slant on that end.

So far so good! Now you want to add in colored bars to show the different spines. Use colors to match your color theme. These are just rectangles with, again, a bit of skewing.

Now for the titles! These are just normal text. Use whatever font you want. The ones further back should be a little smaller than the front ones, but try to ensure they can all be read. Fancy fonts don't work well at small sizes. Keep it simple and legible.

You're nearly there! Select the area of black showing through around all four rectangles, make a new layer with it, color it tan (or whatever color you want your empty area to be) and bevel it. That creates the slight rise around your four book spines.
Voila! You're done!

Let me know if you have any questions! It should take about five minutes to do this once you get the hang of it.
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