The 'Caddies', that strange band of beggars and guides that formed a distinctive feature of sixteenth-century Edinburgh, provide an exciting human flavouring to the vividly realized, physical setting of Mollie Hunter's story of murder, intrigue, treachery and political conspiracy [The Spanish Letters]….
Mollie Hunter has portrayed an entirely credible society in which not all enemies are without honour. This story … will be seldom found on the library shelf.
Gordon Parsons, "Book Reviews: 'The Spanish Letters'," in The School Librarian and School Library Review, Vol. 13, No. 1, March, 1965, p. 95.