De Senectute was a short, cogent, eminently lucid, very practical treatise, one which offers some particularly penetrating correctives for our own day, in which old age is (once again) largely looked down upon and dreaded.
Portions of De Amicitia were very good, though I think one could get away with just reading sections I-IX for a synopsis of the entire treatise. Though sections XXIV-XXVII, on the dangers of flattery for friendship, were also very insightful.
I haven't read De Divinatione yet; I plan to read De Natura Deorum first, which seems more universally applicable.
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Some favorite quotes from these two works:
"Sic vitam adulescentibus vis aufert, senibus maturitas; quae quidem mihi tam iucunda est ut, quo propius ad mortem accedam, quasi terram videre videar aliquandoque in portum ex longa navigatione esse venturus." (De Senec. XIX.71)--notice the similarities to the Apostle Paul?
"Ut enim benefici liberalesque sumus, non ut exigamus gratiam--neque enim beneficium faeneramur, sed natura propensi ad liberalitatem sumus--sic amicitiam non spe mercedis adducti, sed quod omnis eius fructus in ipso amore inest, expetendam putamus." (De Amic. IX.31)--an interesting counterpoint, I think, to the stereotypical (and doubtless largely accurate) depiction of Roman culture as built almost exclusively on a reciprocal system of giving and repaying favors
"Virtutum amicitia adiutrix a Natura data est, non vitiorum comes, ut, quoniam solitaria non posset virtus ad ea quae summa sunt pervenire, coniuncta et consociata cum altera perveniret." (De Amic. XXII.83)--a neat little summation of the entire treatise