They're the same person. Melanie Nilles wrote that M. A. Nilles is her "dark side". She gave her "dark side" credit for writing the book...

I just... the fact that this is a thing that exists fills me with equal parts delight and despair. What a world we live in, when you can co-write a book with your "dark side" and have it published in some fashion that allows people to read it. XD