Below are a list of books associated with KDE 4 development. These may be specific to applications, using the Qt Toolkit, or specifically KDE 4 books.
KDE 4 Development Books
Qt 4 Development Books
Book of Qt 4: The Art of Building Qt Applications By Alan Ezust, Paul Ezust.
GUI Programming with Qt 4 By Jasmin Blanchette, Mark Summerfield.
Qt 4: Einführung in die Applikationsentwicklung von Daniel Molkentin
In general, IRC channels mentioned here are available on FreeNode. Connect to irc.kde.org, and check these channels out for further information.
Courses and Certifications
Audio and Video Casts
Source Code Repositories