Dear Karlito,
It is very nice that you want to read about postmodernism in summer thinking that you will not confront it in fall. I have to share my experience about how I started to build my knowledge on postmodernism.
First read writers who wrote on the founders of postmodern roots. From a sociological point of view, Steven Seidman (the sociologist) writes very nice, and also about postmodernism and social reality. This is going to give you a very good idea about the place of postmodernist thought in social research.
You can also read books on Foucault. You can use Stanford Encyclopedia of Philosophy as well (that is online and free) Jana Sawicki once wrote on Foucault's importance for Feminist Politics. If you check carefully, you can find it in an edited book on feminism. You can read Sawicki as well. She is also very clear.
The most important is that you can not learn postmodernist thought simply reading texts. It is being deployed around us, in activism, art circles, behaviours, design, etc.
Also read Althusser (ISAs) carefully, it is a root of postmodern disscussions of our times.
Best,
C.