With [Royal Harry], the adult reader finds his attention gripped with the same intensity as that of any child. This is an ideal situation for a genuine sharing of its experience.
What are the qualities of the book that contribute to its peculiarly universal character? The magical appeal of the core of the tale demonstrates ancient fealties and gives us hints of treasure to be discovered and a throne to be regained. The reader is refreshed by his journey deeper into the past and closer into the wild countryside untouched by urban and industrial development; but he is not allowed to make his 'escape'. All the happenings of the story are firmly embedded in the reality of human existence, the same then as now, and the same in childhood as when we are grown up. (p. 262)
Much has been said in praise of William Mayne's dialogue. The conversations in this book have many of the characteristics of the give and take of the best kind of improvised drama. They distil the quality of the relationships, and delight us by their dryness and wit. Much too could be said of the incidental details that enrich the fabric of the story's reality.
'Have you read Royal Harry?'. The question from teacher to pupil will indeed be the same question he will ask his friends. They will relish it together. (p. 265)
Pat Smyth, in The School Librarian, September, 1971.