The Junior Bookshelf
[The Spanish Letters] is a good cloak-and-dagger, the plot moving at a fast pace, the characters reasonably well-developed for this type of book. There is plenty of suspense, although the usual kinds of trick are employed, the holding hostage of a beloved only daughter, and the boy hero's discovery of a secret passage to release her.
"For Children from Ten to Fourteen: 'The Spanish Letters'," in The Junior Bookshelf, Vol. 28, No. 6, December, 1964, p. 381.