GPU Computing Gems Emerald Edition:
--- Should be a fantastic resource.
Programming Massively Parallel Processors:
--- This introduces you to the concepts of parallel processing and terminology for the GPU. It spends a lot of time (the first several chapters) explaining concepts in CUDA, but the way of thinking is similar to OpenCL.
OpenCL Khronos specification:
--- This thing is your bible. Don't leave home without it.