Gabel gives the principles for the abridgement in the Foreword:
The principle guiding the selection of passages for this abridgement was initially that the content be at first hand, the Pope writing either as observer of or as participant in the events related....A further reduction was effected by cutting or omitting certain blocks of subject matter of interest mainly to the specialist and available to him in the complete edition. The author's fondness of repetition and for lengthy speeches offered still another possibility for abbreviation, though care has been taken to preserve characteristic examples. (p. 11)
The whole Commentaries primarily covers the years 1458-1463, but in fact cover a lot going back to 1405 and even farther in history, since the author often goes back and reflects on his life and the historical course up to it, but the abridgement for practical purposes can be said to cover the period of the Renaissance from the Council of Basle up to Pius II's death, at which point he is preparing to lead personally a crusade against the Turks because he has been unable to get the major leaders of Christian Europe to recognize that the Ottomans are reaching the point of being able to invade Europe itself.
The edition I have is a nice reprinting in 1988 by The Folio Society; it's a cream-color book with a red titling leather on the spine. The covers are in what's often known as 'elephant hide', a parchment-like treated paper that is very durable, and it is stamped with the outline of the coat of arms of the House of Piccolomini. The text itself is in Centaur typeface on what's known as antique wove paper. The book also has a set of plates of paintings commemorating various things in the Pope's life.
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Pius II, Secret Memoirs of a Renaissance Pope, Gregg, tr., Gabel, ed., The Folio Society (London: 1988).
