The following are my notes from reading Chapter 1 of Matthew's book.
From chapter 1 of Matthew Casey's The Return
At that time his voice shook the earth, but now he has promised, “Once more I will shake not only the earth but also the heavens.” The words “once more” indicate the removing of what can be shaken —that is, created things—so that what cannot be shaken may remain.
The overall lesson from the opening chapter of Matthew's book is do not despise the shaking because God is going to make make things happen. Chapter 2 is about "Missing: The Presence of God."