Read 1 of #1, #2, #3, #4 from OS Readings or something similar.
Note that although there are only 2 copies on reserve in the library, these articles came out of journals and thus you can find other copies in both Sprague and the CS library.
These papers cover some operating system environments, more from a user level than an internals level, but still worthwhile. If you have already read one of these, then read something on processes. Look at the bibliography at the end of the process chapter in Tanenbaum.