To be honest, for the target audience of this book, CUDA is the only thing that matters right now. OpenCL and DirectCompute will possibly be significant in the future if AMD and Intel can catch up with NVIDIA in GPGPU performance.
Here's a question you might be able to answer... what exactly do Nvidia cards do better than AMD cards? In the bitcoin and litecoin worlds, Nvidia is horribly outclassed for hash algorithms like sha-256 or scrypt. In the gaming world it's closer, but ATI's 79xx series still wins. (Especially when you consider in the cases a single 7970 isn't enough, which is true for several games, you can get 2 7970s for less than the price of a GTX Titan and win that way. Or a single 7990.) 32bit and 64bit floating point benchmarks favor AMD, the openCL Sala and Room benchmarks in Luxmark favor AMD... (Not exactly fair for that one since it's not CUDA, but the difference is enough that a performance boost from CUDA likely wouldn't close the gap.)
I suspect Nvidia's advantage is their proprietary software like PhysX and other software used in high-end computing or scientific-computing, and possibly they scale better (or simply there are Nvidia-supported solutions) when you want to add dozens of cards together. Is this the case? Because I don't see how one can claim AMD needs to "catch up" in performance if you're looking solely on a card-by-card basis.
