Ideagirl,
Basically, I just wanted to list good books (in my opinion) for middle grade readers. I thought using the back cover copy would give the best idea of what the book is about.
You would have no trouble getting that permission from the publishers for that kind of usage. However, you would need permission: reproducing the entirety of a book's back cover copy is not considered "fair use."
You can quote from the back cover copy as part of "fair use" in writing a review or similar, but reproducing the entire text would not qualify. Now, I suppose you can do it and just deal with the consequences if anyone complains (like Momento Mori is doing): as long as you're ready to take stuff down if anyone has an objection, you won't have a problem.
But in theory, the back cover blurb is text copyrighted by the publishers, and therefore can't be reproduced in full without their permission.