No idea what was mentioned last time but:
Sol Lewitt's "Sentences on Conceptual Art" is brief, elegant, and eternally relevant.
Pretty much everything Kandinsky has ever written - particularly regarding spirituality and color. He's probably the most lucid and inspired writer among any 20th or 21st century visual artist
Guy Debord's "Society of the Spectacle", which only seems to become more and more relevant as media becomes increasingly omnipresent and disposable. You know a great theorist from a hack academic when you can simply hand any stranger on the street one of their texts and have it resonate with them. "In societies where modern conditions of production prevail, all of life presents itself as an immense accumulation of spectacles. Everything that was directly lived has moved away into a representation."
On that note, Dick Hebdige's "Subculture: the Meaning of Style" is interesting to read these days, as so many classic subcultures are being folded upon each other and churned around through the internet.
Borges' "Blindness" where he meditates on the progressive loss of his vision due to a degenerative disease. You might nitpick about sound artists or something, but in art we're really in the business of looking, and it's fascinating to read such an excellent writer on the topic of vision. It's also like eight pages long, so if you don't read it you are kind of being an ass.