I don't know a great deal about the mathematical formulas with which physicists quantify their study of matter and its motion through spacetime, but I do know that to translate the substance of what those formulas reveal into plain English to non-physicists can be daunting.
Both Hartnett, in this book, and Russell Humphreys in his book, "Starlight and Time, Solving the Puzzle of Distant Starlight in a Young Universe", do only a mediocre job of translating formulas into language. So if you are looking for an easy read on how, if the Earth is young (about 6000 years old), we can be seeing, here on Earth, starlight that appears to have traveled millions of light years, than you will disappointed in this book by Hartnett. However, if you are willing to be patient and do some work, I think the reader will gain a great deal from this book.
I am impressed by, and deeply grateful for, the work Hartnett (and Humphreys) have done, and am able to grasp the broad outlines of their arguments. Perhaps it's too much to expect that in either Hartnett or Humprhreys we should find a person capable of penetrating to the truth about the age of the Universe in a natural science like physics, and be capable of explaining it to lay persons with equal brilliance. I am not aware that Einstein was any genius in explaining his theories at ladies' coffee klatches.
What is needed now is for someone with a gift for communication to find a way to make Hartnett's (and Humphreys') theories easy for the lay person to grasp. I know both have made tremendous efforts to do this, but all those efforts far short, including this book.