The author, Peter Zeihan, does a good job of melding geography, demographics, energy, and food supply to come up with a deterministic approach to geo-politics.
I have no clue how accurate it is, but it's a fun mental algorithm for simplifying complex and nuanced situations.
His prediction, and I'm paraphrasing here, was that Russian Demographics are more or less in free fall. Every year after ~2020 the Russians have less and less fighting age men and their army will be weaker and weaker.
Additionally, because most of populated Russia is a flat plane, they have no geographic barriers to protect their cities. Therefore they need to forward position their troops in order to feel secure.
So his assertion, was that based on previous Russian military conflicts, Ukraine was the next most likely target, and that if Russia wanted to do anything, they'd have to do something sometime earlier on in this decade. Otherwise they can't field an army of significant size to forward position their troops.