Check out some books by Suzi Gablich (Has Modernism Failed and The Reinchantment of Art) and Lucy Lippard.
There are many other books on this shift available but those are two very good authors.
Definitely read some books on philosophy as it underlies the ideas from which the art was created. You really won't understand the art without a strong philosophical and literary bacground first. If you are really serious about seeking this information here's a good way to begin.
You can do some very good research by making yourself a chart broken into three columns. Column 1, PreModernity; Column 2 Modernity; Column 3 Postmodernity.
Then begin to do your research to list the things that will fit into each section - you'll then have a visual that can give you a comparison and contrast to look at. From there, you can begin to ask the questions that will lead you to the artworks/artists that fall into the different time periods. You'll not be able to figure out who/what is postmodernity, without an understanding of the two time shifts that preceeded this one. And, be prepared to come out of this process with lots more questions. You have the possibility of going on a very exciting adventure here.
Good luck in your pursuit of knowledge.
Here is a link that might be useful: Lynda Lambert Art Gallery
