I would say YES, most people can become a great mastering engineer. I say this because most of the mixes I receive from clients are pretty good and I know the audio engineer (the band member with a computer) has only minimal training. He could easily be great if he put a little more study and practice time into it. And if he had this book to teach him what took me over 17 years to learn! Ha!
The bigger question is, how much time and effort are you going to put in? You know, everyone can be in good physical shape, or at least in decent shape. The choice is yours. But, is there going to be a diet and exercise plan that's followed daily for months, or is everything going on eBay or craigslist a few weeks after January 1st? It's up to each individual to do what's necessary to achieve their end goal.
You get out what you put in, even when it comes to audio mastering. This book gives you the knowledge and tools you need to become a great audio mastering engineer. And you'll learn a lot faster than I did. But it's still up to you to read this book in its entirety and put in the practice time.