The Project Management Institute is the industry watchdog for this discipline and the publisher of your textbook A Guide to the Project Management Body of Knowledge.

Gantthead.com, "the online community for IT project managers," has different levels of membership. At the most basic (free) level, you can read lots of current articles and access some of its project management templates.

ProjectManagement.com is run by Gantthead and also has plenty of articles and discussions.

JustProjectManagerJobs.com is a project manager-only employment site managed by JustTechJobs.com.