Habakkuk covers some really important themes:
God's sovereignty
"For behold, I am raising up the Chaldeans [to punish Judah]...But they will be held guilty" (1:6, 11). This is one of the things about God's sovereignty that repels people from believing this doctrine or even Christianity altogether but nevertheless it's Biblical. Compare Romans 9:11-24 and Romans 3:19 and Ecclesiastes 12:14.
Righteousness through faith (2:4)
Paul expanded upon this in Romans and Galatians, Martin Luther was saved through Paul's explanation of this doctrine which was a major cause of the Reformation, and everybody's eternal joy relies upon their understanding of this doctrine.
God's glory (2:14)
Reverence for God (2:20)
God is eternal (3:6)
God is so everlasting that ancient hills collapse and perpetual mountains shatter in God's presence.
Habakkuk, the man, knew that only God could satisfy his deepest desires (3:16-19).
Habakkuk is a man who seems to have no feelings hidden from God. He reminds me of the psalmists David and Asaph and the apostle Peter.
Habakkuk is a book that can be read in one sitting. That makes it easier for me to understand.