If the author of the summary, (whether it was Morrison or someone else) is not identified under the summary, it is more than likely just an uncredited summary by someone who works for the publishing house.

Since there are a number of different editions of "Sula" it is impossible for me to give you direct advice as to what the back cover actually says. But you do need to cite it if the words are not your own.

Therefore, whether you were using a direct quote or summary, you would then do something like this (just an example): The story of Sula is a tale of a young girl who must overcome obstacles of racial prejudice and a distant mother to find her own voice (Plume Press).

You might also want to check MLA for other citation needs for both internal and works cited reference. The link is below.