Cecil papers
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"The Cecil Papers are a privately held archive of approximately 30,000 sixteenth and seventeenth-century manuscripts, consisting principally of the correspondence of William Cecil, Lord Burghley (1520-1598) and his son Robert, the 1st Earl of Salisbury (1563-1612). These two men dominated the administration of government during the reign of Elizabeth I and the first eight years under her successor, to the extent that critics suggested that England was becoming a regnum Cecilianum.... In addition, the collection contains documents acquired by Robert Cecil that had belonged to his rival, Robert Devereux, the 2nd Earl of Essex.The papers span the period 1520-1668."
(from publisher's description, viewed Feb 16, 2014)