This is book number 6 in the Swallows and Amazons series.
The crew's on holiday, and they turn their energies to mining for gold, aided by pigeon messengers Homer, Sophocles, and Sappho. The adventurers comb the nearby hills for a fabled lost claim, while being shadowed by a mysterious figure they dub "squashy hat." Undeterred by drought, sudden brushfires, and the continuing presence of Squashy Hat, the young prospectors persevere in their quest--with surprising results. Full of the dangers and dark adventurers of old mines and forgotten claims, Pigeon Post has an irresistible appeal to the persistent explorer in every child.
"There is plenty of excitement, a little danger, a quality of thinking, planning and fun in connection with a gold-mind. The ingenuity of this group of children is delightful and stimulating." --The Times Literary Supplement
"These books about the four Walker children and the two Blackett girls, who spend their vacations in the English Lake Country sailing and camping on their own, are truly first rate." --Philadelphia Inquirer