The CEP website is an excellent source for more material, at http://www.complexevents.com/
There is also a discussion group on Yahoo for CEP, see http://groups.yahoo.com/group/CEP-Interest/
The Event Stream Processing resource site http://www.eventstreamprocessing.com/
Read about CEP in WikiPedia at http://en.wikipedia.org/wiki/Complex_event_processing
Read about ESP in WikiPedia at http://en.wikipedia.org/wiki/Event_Stream_Processing_%28ESP%29
The Power of Events by David Luckham, from Stanford University, a book on CEP.
Esper Release 1.0 news announcement on InfoQ
Blogs about CEP and ESP..