You can purchase it here: http://www.thechiprack.com/
David Spragg, one of the authors, runs this website you're on.

If you want to wait a few months, they may publish a 16th edition prior to the convention in June. (Hasn't been announced re: 16th edition yet, though.)

As to why to buy it -- they update values each year, including increases and decreases in chip values.
It can pay for itself easily if you're looking to acquire more expensive chips.
Sometimes, a new find of an old chip occurs, and it's important to know when such an event changes values.