Burckhardt The Civilisation of the Renaissance in Italy by Jacob Burckhardt. Massive reference work on the period - important for the beginnings of Christian and Hermetic Kabbalah. Galileo Project Entry point into the Galileo Project's massive store of information. A superb historical resource for the Hermetic historian. This is the Plato Page.
All philosophy is footnotes to Plato.
The Gnostic Society Library Highly recommended. Once were lost but now am found, as the song goes. More essential pseudepigrapha and apocrypha. This great little essay is the history of its glacially slow combustion. Another useful entry-point to classical source material.