There are a couple reason why recommendation may tend to bias to older books, regardless of whether or not the older books are better.
One is that the longer a book is out the more people may have had a chance to read it. Which implies it is likely that less people have read the modern book, and thus less people would be willing to recommend it.
The other is that you will find older results that predates many modern books. They could not possibly have recommended a book that was not out yet.
Plenty of OpenGL books will cover some topics of computer graphics, and many books on computer graphics will have examples with OpenGL. And the OpenGL code might be obsolete.
However, focusing on the computer graphics, it is a safe bet to start with an older book. The math does not change, but new algorithms are developed. It often helps to understand the older algorithms to understand the recent ones. Plus, that a book is newer does not mean it has more advance contents.
About Computer Graphics C Version, it is certainly dated. It goes on and on about old hardware, not a problem, skip that. It is not focused in games. However it covers a lot of 2D and 3D graphic algorithms that you'd miss with the other book. If you want to learn the nitty-gritty of old graphic algorithms, this book is great. It covers the essential math in an appendix.
About Essential Mathematics for Games and Interactive Applications, I have the second edition, it does a great job covering the essential math (it is in the title after all), and it goes straight to 3D. I find this book more practical, it will get you started much faster, but it also seem to fall short, staying too much in the essential. There is a little OpenGL code in this book, those can be ignored.
My recommendations is Physically Based Rendering: From Theory To Implementation. This book cover some not so essential math (it assumes you understand some basics), and will go into implementing the pbr as the title suggests among other modern shading techniques. Consider it a complement to either of your picks. There is a free version online.
And of course, if you run into something you don't understand, we have the stackexchange network.