Just to make your researching easier....
Thanks to Katie, the consummate writing center researcher, here are those valuable links to start surveying the literature about writing centers:
Writing Centers Research Project http://coldfusion.louisville.edu/webs/a-s/wcrp/
Writing Center Journal Digital Archive http://louisville.edu/a-s/writingcenter/wcenters/wcj.html
Writing Lab Newsletter http://writinglabnewsletter.org/
PeerCentered blog http://bessie.englab.slcc.edu/pc/
I'm also fan of just visiting our Nicholson 10 library (where you'll find WC books as well as past issues of WCJ and WLN ) or visiting the PE 1404 stack in Wilson library (what ...real books-- not downloadable PDFs?).