Since its initial mainstream popularity and impact on the sound of contemporary R&B, neo soul has been expanded and diversified musically through the works of both American and international artists. Its mainstream presence declined during the 2000s, although newer artists emerged through more independent means of marketing their music. In his book "The Essential Neo Soul" (2010), music journalist and culture critic Chris Campbell writes that, while the genre has been "woefully misunderstood and its artists mis-marketed", there is "a historical and social relevance that validates its designation as the current face of alternative progressive soul music (in both underground and overground circles), complete with a distinct origin and developmental evolution". According to Mark Anthony Neal, "neo-soul and its various incarnations has helped to redefine the boundaries and contours of black pop."