Whilst trying to remind myself of the word "Sfumato" (the technique for portraying voluminous fields of light and dark by layering smokey veils of colour - something of a challenge for most of us screenprinters), I was lead by this gem of the web...
The Notebooks of Leonardo Da Vinci - Wikisource
I thought I'd share - warning! - no pics! its a lot of words... I've not read even a whole chapter yet but skipping around between sections and I've already enjoyed some great insights that will stay with me till I die!
Its like the Richie Goodtimes halftone school x a zillion - you ought to take a look.