The short version: see all the links for these two books and their related resources on our books page, at the top.
I haven’t seen these books yet – I look forward to doing so. GPU Zen 2 is the next in the series of ShaderX/GPU Pro/GPU Zen books edited by Wolfgang Engel. Sixteen (or seventeen, depending how you count ShaderX2) books since 2002 – amazing. BTW, I maintain a links page for just this series of books.
The Ray Tracer Challenge I don’t know anything about, I just happened upon it – I’ve just ordered one. Two things I like seeing: First, the YouTube advertisement reminds me a bit of “Welcome to Night Vale.” Second, if you buy the physical and (DRM-free) eBook together, the bundle is cheaper. This should be the norm. Unfortunately, Amazon’s price is $10 less than the publisher’s, so, much of that savings gets wiped out (though Amazon has just the DRM’ed Kindle version, of course).
Both books are selling well, but neither can outdo the popularity of Plant Based Cookbook for Beginners, #1 in Best Sellers in Rendering & Ray Tracing. (More meaningful is the new releases listing, but that’s not as entertaining.)