Hi Kelly-
Great question. Copyright can be confusing, especially when it pertains to fair use (which seems to be what you're asking about), but I'll talk you through some of the guidelines.
Copyright extends to any reproduction of a work, including reproduction on websites. If you want to to direct your wiki users to content that is freely available online, you can link to the page where the resource is found, but you wouldn't be able to copy/paste it to your wiki without permission from the copyright holder. If you wanted to provide a citation to an article in a database, you could also do that, but you couldn't (for example) take something from behind a paywall and post it on your own page. If you're creating PDF's from books or magazines and posting them on the internet, you're likely to be running afoul of copyright laws. If you are linking to PDF's that the copyright holder has made available to the general public, you're fine.
Fair Use does provide for certain kinds of use of copyrighted work, and would extend to something like quoting short passages for purposes of discussion, but reproducing an entire chapter or article in its entirety would be a fairly clear violation. It almost sounds like you're talking about an online version of a course packet (which you might be familiar with from college), but materials in course packs are technically required to be cleared for use by their publishers before being distributed to students. Publishers do sometimes charge for use of coursepack materials even in educational settings.
The Stanford University Library has a fantastic page about Copyright and Fair Use, the following is a link to their explanation as to how these apply to websites:
http://fairuse.stanford.edu/Copyright_and_Fair_Use_Overview/chapter6/index.html
..and info about coursepacks, which might be helpful:
http://fairuse.stanford.edu/Copyright_and_Fair_Use_Overview/chapter7/7-a.html
You CAN post materials in their entirety if you obtain permission to do so from the copyright holder (usually the publisher, sometimes the author). Some publishers are happy to grant permissions, some not so much. Info about requesting permissions (again, from Stanford) here:
http://fairuse.stanford.edu/Copyright_and_Fair_Use_Overview/chapter1/1-a.html
I hope this helps!
open or closed access
One point you may want to consider in making a fair use argument: is your wiki openly accessible to the world? If you password-protect the reading materials so that only authorized users can view them you will have a stronger fair use case.