I started with his logo. 

 

As you can see in the photo above of the back cover, here they have crammed the whole company name into that small white circle surrounding sword guy. I wanted the circle gone but figured that would change things too much. So I eliminated part of it as you'll see in the example below.

 

The font that is used in the company name you see below, was crafted specifically for this book series.

So I modified the logo. But there was more I wanted to do. 

I spoke to my cousin and we had a good long chat via FB messenger. I shared that the green and yellow that was used reminded me too much of Zelda, the video game.  I like that they had added in a screen of one of the many drawings done by their artist (they did not have this on Vol. 1)  

My cousing explained that the colors were important to the whole series and that they needed to be green, red, blue and black. Also, he liked the idea of the aesthetics of the colors so that a person who has colledted the entire series, having them lined up on a bookshelf, and seeing at once, these books were a "set".

Fonts. The same font was used over the entire cover; it was too much and heirarchy was sketchy. I kept the specially made font for the key pieces and used Baskerville bold for the sub key and Baskerville itallics for the back cover story.

I mocked up the following 4 covers in about an hour with my cousin's illustrator's graphics.

In the end, there was no way he could make these adjustments any time soon due to expense of redesign. So he asked if there was ONE thing I could change about what he currently had, what would it be.

 

My answer: The font.