I think that a case can be made for Pearce's work to be considered amongst the rare air of 20th Century Children's literature masterpieces.  The metrics will always be debated as to what constitutes such a work.  Yet, I think that some of the elements and importance of Pearce's work helps to make it move into that high level of stratification.  The establishment of the work as a critical component of the children's literature canon is represented by the spirit of magical exploration that the garden represents as well as its statement on growing up and maturation.  These themes are critical to its established place in the realm of great literature for children.  At Pearce's death, the placement of her work in this location was already understood:

Describing the work as a whole, the respected critic John Rowe Townsend states in his 1965 book Written for Children: 'I have no reservations about it. If I were asked to name a single masterpiece of English children's literature since [the Second World War] . . . it would be this outstandingly beautiful and absorbing book."

The metrics will always be debated.  Yet, I think that one can see that the work can and is considered to rank amongst the very best of children's literature.  Its spirit of possibility and hope that seems to be intrinsic to children and the process of growing up are reasons it is placed there.  When children's literature is scoured for what is said about childhood, Pearce's work makes one of the most compelling statements.  This is what helps to make the work a masterpiece of modern children's literature.