Currently on Amazon for the Kindle, a great free that is available is "Evernight". Includes bonus material and is a pre-order! You can't get "Evernight" on the NOOK sadly and is the original version but also $7.99 for that ebook.
Also for both devices, you can pre-order for $0.99, "Cloaked", which has bonus materials for the NOOK. It'll include a sneak peak into the next book by Alex Flinn, "Bewitching". This offer is good till January 31st! So get it will you still can!