Bit weird...guessing it's a special edition? Seeing as there are 2 discs in this set. It was probably their most popular blu-ray release to date (at a guess).
I think the Nas: Time Is Illmatic steelbook will be more worth it. I'm a casual Nas fan, and it doesn't seem to be getting a regular blu-ray release so I might go for that (unless it's silly money).
BTW, Dogwoof are one of the best UK independent companies. Especially for documentaries. I'd recommend picking up The Act Of Killing and Leviathan if you want to see something groundbreaking.