I know time may be limited for you, but if you have ~1-3 weeks to really learn CUDA, I highly recommend CUDA by Example: An Introduction to General-Purpose GPU Programming (Amazon.com). It does a fantastic job of explaining the general concepts of GPU programming, and does a great job of getting one up to speed with NVIDIA's GPU language, CUDA.
CUDA is arguably one of the most widely-used GPU languages—certainly within the high-performance computing community (e.g. NASA). It gives great control over lots of details, but for many tasks, it also requires low-level knowledge of the graphics card. It's not something you'll learn in a day, probably not even in a week, but it can be worth it.