-Images are relatively plain and there is a lot of potential for text placement and they work well as both landscapes and portraits.
-You could definitely find a way to eliminate the black box. Perhaps, choosing a bolder typeface and adding a drop shadow. Though, the fire and water images might be more difficult to eliminate the box from, if you edit the image by darkening or blurring a portion the background for example, it might work.
-Contrast of white text works really nicely on the green water image.
-Consider typeface choices. There is nothing wrong with it, but maybe there is one that relates more to your content.
-Type treatment is nice for the book/author.