Not to sound too much of a suck-up, but you can't go wrong with Julia's "Ultimate Cookies" book! I love the beautiful artistry of all the cookies and the gorgeous photos. It's eye candy! I've spent many hours just ooooh and ahhhing over all the wonderful cookies.
I also like Autumn Carpenter's books. Her "Cookie Boxes and other 3D Projects" is more of a pamphlet than a book, but I enjoyed her simple instructions. The projects are very approachable, especially for the beginning decorator.