It is much easier to write a book description for a book that is simply a collection of prose, or poetry, etc. Writing a book description for a novel is no easy task. You don't want to give away the whole story but you have to give a bit of the plot in the description and do it in such a way that it is interesting to the reader. You want the reader drawn in.
I've been trying to write a book description for my first novel. I've been rewriting it over and over yet it still feels unfinished to me. I feel that more editing needs to be done. Maybe the reason it is not coming together as I'd like is because the novel is still a work-in-progress. Maybe it's best to wait until it is complete before attempting to work on the description again. But if ideas come then there is no choice but to give it another shot because it just might bring the book description the needed improvement.