Location of Smokescreen 20

Personally, I have never seen where this book has dropped. Sources and forums say Zakum3. But I was wondering if there is another location. If not, how damn rare is this book? Out of all my Zakum runs, I have not seen one drop...

Yeah they drop from Zakum. I'm buying them all of the market when I see them in a store, I have 4 now... which I will most likely never use though... I just use them for my personal collection...