Websites for professional associations closely tied to your field of interest can be great sources of information, but also keep in mind that at times such organizations may offer you membership-only content.
If you come across an association selling a book title where you’d like the librarian to do a rush order, contact him at firstname.lastname@example.org (315) 443-3715 .
In PR, one has the Public Relations Society of America - http://www.prsa.org/ - which – for college students would mean membership in the student branch of PRSA known as Public Relations Student Society of America – which does maintain a Syracuse University Chapter
Keep in mind too that any subfield of PR you explore will have its own professional associations as well.
Plunkett Research Online - is a business database SU Libraries offers that – in addition to industry related market research reports – also links out to lists of relevant professional "associations" within various industry areas.
Leadership Connect - In addition to being chock full of directories for government agencies and officials and lobbyists, etc. - there are also corporate, non-profit, healthcare and media directories available in this enormous directory database. "Leadership Library on the Internet" also includes a general directory of "associations" of many types.