GPU Computing Gems Emerald Edition:
http://www.amazon.com/GPU-Computing-.../dp/0123849888
--- Should be a fantastic resource.
Programming Massively Parallel Processors:
http://www.amazon.com/Programming-Ma...d_bxgy_b_img_c
--- 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:
http://www.khronos.org/registry/cl/
--- This thing is your bible. Don't leave home without it.