those big ass volumes "art in theory" and "Theories and Documents of Contemporary Art", i would think are great conpendiums of writing organized in an easy to use fashion. i've read only parts of both, always meant to go from cover to cover but you know how it is...
There's a good new general one out by Thames and Hudson - Mirror of the World: A New History of Art by Julian Bell.
Obviously lots of great monographs - Phaidon has done a great job on upping the ante on the quality of reproductions.
Though Hughes was great on Goya, Uglow on Hogarth, Schama on Rembrandt, Richardson on Picasso, lots of others as well nailing periods or individual artists. The aforementioned Berger on Picasso is also rather fine. I really liked Kathleen Raine on Blake as well.
Hope this is of some use.
what exactly you teaching, modern or ancient or what?
I dunno, whats your idea of modern?
Also try "Inside the White Cube." Can't remember who wrote it but it's easy to find.