MANGA
The series is seventeen books long in Japanese and counting. So far in North America there's been thirteen released. I recommend talking to your local anime store to see if they have a membership you can sign up for and get discounts everytime you buy books (that's the cheapest way I've found to keep up). Of course, there's scanlations but they're hard to find, unless you dig around somewhere on IRC. If anyone has information pertaining to the scanlations, or better yet the zip files of the scanlations themselves I'd be happy to host them. An alternative option is some big name bookstores offer memberships and 10% discount on the original price. I've found them to be cheapest without a membership at Amazon.com though, beating out most competitors with their discount cards. Below are links you can follow to purchase the books if you so wish.

NO IMAGE AVAILABLE VOLUME 13