As someone else pointed out non-mainstream is in the eye of the beholder. It also may be affected by where you live. I have my favourite authors, and they're generally a big deal and very popular over here (I live in the UK). So I pretty much wanted people to think of authors that perhaps weren't as popular as they should be, or maybe up and coming authors on their way to being best sellers.

Personally, I've never heard of Nalo Hopkinson, but I'm certainly interested in checking her out based on your comment! Thank you. :D