I bought this book for a course on child development and found it to be a very useful resource. Mercer explains things simply and clearly, illustrating his point with examples of actual dialogue recorded in learning situations.
Written from the social constructionist perspective, it incorporates Vygotsky's idea of the zone of proximal development and Bruner's scaffolding. The latter is more likely to be known to teachers and those interested in the psychology of learning. The book takes the view that both teacher and learner contribute to the learning process in a collaborative manner, at least when things go according to plan.
As a former school inspector, AST, school leader and behaviour consultant I have always held this book in high esteem. For me, it explains the need for dialogue in the classroom in a way that supports both new teachers and 'old'.