The Courts of the Morning by John Buchan

The Seaforth flew first to Loa, where Archie asked for news of Don Luis. He was believed to be on the southern front, somewhere in the Pacheco area, where there was a good deal of activity. Loa itself was at present stagnant, a mere blockhouse to guard the road to the Courts of the Morning, and a forwarding depot for Magdalena. Lossberg's advance party, which a week before had been within twenty miles, had now withdrawn.

Then, hugging the skirts of the hills, and having a good deal of trouble with the eddies of wind that blew down the gullies, Archie flew southeast to Magdalena, Escrick's headquarters, under the snowy peaks of the Spanish Ladies. There he had an interview with Escrick's chief Intelligence officer and was shown the dispositions of the enemy in the Tierra Caliente. The nearest enemy planes were based on the Mines, and were probably at that moment busily engaged, since Sandy was worrying the Universum sector. But there was no one at Magdalena who had any knowledge of the approaches to the Poison Country; it was out of their area, and belonged properly to Peters.