I have a lot of books on my bookshelf (on hold) that are wishlisted by other members. Why are some of them marked with a red W and some with a green W? What's the difference?

A red W means they are on someone's Wishlist.  A green W (with a little A behind it) means the person has it on autorequest.  If you were to post the book, it wouldn't go through the process of waiting to see if the person still wants it.  It would be immediately snatched up.

The green W should have a little "a" next to it ~ that means it's on autorequest by the next member.... If you post the book, the system will automatically request it for the wishing member (no waiting 48 hours for them to respond).

The

A red W means that it is wishlisted and the first person in line for the book has it on 48 hour hold.  A green W means that the first person waiting on the book has it on auto-request. 

Thanks, everyone!