In talking to another artist today, I noticed that she had included a reading list on her website and thought it was a very good idea. I hope you dont mind, Amanda, if I steal it!
This is just a starting point - I will add more - but this is what I have been reading most recently.
Willem De Kooning - The Late Paintings, the 1980's, Ed. Janet Jenkins, 1995 ISBN 0-935640-47-9
Ways of Seeing, John Berger, Penguin, 1972, ISBN 978-0-141-03579-6
Art of the 20th Century, Volumes 1 & 2, Taschen, 2005, ISBN 978-3-8228-4089-4
Paths to the Absolute, John Golding, Thames & Hudson, 2000, ISBN 0-500-28359-1
The Object Stares Back, James Elkins, Harcourt, New York, 1998, ISBN 0-15-600497-6
What Painting Is, James Elkins, Routledge, London, 1999, ISBN 0-415-92662-9
Concerning the Spiritual in Art, Wassily Kandinsky, Tr. MTH Sadler, Dover Publications, New York, 1977, ISBN 0-486-23411-8
Colour in Art, John Gage, Thames & Hudson, UK, ISBN 0-500-20394-6
The Art of Richard Diebenkorn, Jane Livingston, Whitney Museum of American Art, New York, 1997, ISBN 0-87427-107-X
Drawing and Painting People - A Fresh Approach, Emily Ball, Crowood Press, UK, ISBN 978-1-84797-088-6
There are many more I could list, but not enough space; these are just a few.