The ministry of the Conde Duque de Olivares (1621-1643) is a key period in the history of modern Spain. Between 1978 and 1981 Professor John H. Elliott, who at the time was preparing his well-known political biography of the Count Duke, published with the help of José Francisco de la Peña two volumes with some of the most important state papers and letters. The book, carefully annotated and with precise preliminary studies of each document, soon became an essential source for understanding this period. The original edition of Editorial Alfaguara has been out of print for a long time, and now a new print of the two volumes is presented, here incorporated into one, widely revised and updated by professors John H. Elliott and Fernando Negredo del Cerro. It is a fundamental work, of permanent value for the study of seventeenth-century Spain, and which will shortly be complemented by a new volume with the correspondence, hitherto unpublished, of Count Duke with Cardinal Infante.