Columbia subject headings:
Onís, Federico de, 1864-1936--Correspondence.
Unamuno, Miguel de, 1864-1936--Correspondence.
Unamuno, Miguel de, 1864-1936.
Harvard subject headings:
Unamuno, Miguel de, 1864-1936--Criticism and interpretation.
Princeton subject headings:
Unamuno, Miguel de, 1864-1936--Criticism and interpretation.