As far as I know there's not any hard rules regarding info compied from the web. I usually try to add credit if it's anything other than the blurb on the back of the book. For some guidelines, you might want to check out this thread

http://www.paperbackswap.com/forum/view_topic.php?t=60867

and if you need anything, you can always PM Sherry for help!

The only thing to be truly concerned with is whether the information you are copying is from the editor/publisher/author or another source intended to be quoted when referenced. Sites that give a generic description of a book, such as Fantasticfiction or StopYoureKillingMe, or RomanticWriters, do not care if you use their blurbs, that is what they are there for, as is the info from the editor/publisher/author. But, when you get info from any site and it is marked as an OPINION/REVIEW then they intend for you to quote them, and you should always do so. It is actually preferred that descriptions be that, descriptions, rather than review/opinions, but sometimes, that's all you can get, and it has to do, but it really needs to be quoted to eliminate any possiblity of legal repercussions. This is true whether you are putting the info in the description or in your review for the book.

I hope that answers your question well enough, and as Alana says, you can always PM me directly on any questions regarding what would or would not be approved when updating a book. I've tried to include in the post she listed anything that has been made policy as well as anything I've encountered while working on the data edit team.