so reading the spec is a good start, its well written and you can skip the boring parts. After taking a look at AMD's university kit and Hello JOCL you should be ready for action. The junit tests cover the public api... so feel free to take at them too.
which is in my opinion the best introduction to parallel programming you have to port the cuda code to opencl though but still as there is quiet a lack of opencl literature these two helped a lot
but must say it was quiet disappointing this seems to be more a function reference than a proper introduction to opencl and gpu programming hope this one will be better: