I bought the campaign setting book for its background/detail on the BC world, and thought it was well worth the money. The Amazon reviews may be helpful: https://smile.amazon.com/Black-Company-Campaign-Setting/dp/1932442383/
If you do decide to buy it, I would recommend getting it direct from Green Ronin, as the copies on Amazon tend to be pretty pricey and GR has slightly-dinged copies still for sale at $30. https://greenroninstore.com/products/the-black-company-campaign-setting-scratch-dent
This is just straight up wrong. It's a coinflip on what is going to be popular and there are tons of confounding variables. This book goes into detail about it.