Note the colors from the front cover are being continued on the back. Additional color is added with the author photo and the various text colors.
Note the number of impressive reader reviews on the back cover. Not something that a beginner can count on, but helpful for sales if at all possible.
The scene from the front cover is repeated on the back cover, except for the figure of Martin Luther. This gives consistency of color and design.
The balloon bubbles on the back cover provide an interesting visual effect, while also helping to get the written message across to potential readers.
Here's a good example of a dust cover jacket for a hardcover book. Note that with the flaps there is plenty of additional space for more sales copy.
Featuring a fictionalized representation of a Russian train on the Trans-Siberian Railroad. Book cover reproduced with permission of the author.