The book of Nahum is really about hope for the people of Nineveh and how they have run out of time to repent and return to God. Jonah was sent with a message for them a century and a half before Nahum's words were written, but Nineveh did not heed the warning and was now staring down the face of certain judgment. It's at this point that you might think, "But God is a god of love, how could He destroy Nineveh?" It's true that God is a god of love, but His love doesn't override His wrath. God hates sin (though He loves sinners) and does what He needs to do so that His glory is known throughout the earth. This sounds selfish unless you understand that God is the greatest being in existence and anyone else's glory falls short of the ultimate glory of God. God is slow to anger (he waited 150 years to do this), and He is quick to uphold those who place their trust in Him. The sins of Nineveh were great, so their destruction was anything but unjust. Those who placed their faith in the Lord then as in now, were saved.
The real lesson we take from this book isn't much different than many of the other lessons we've drawn throughout the Old Testament: are you placing your faith and trust in what Christ did on the cross? Then are you seeking to honor God as best you can with your life? If so, then you have no fear of destruction such as was faced by the Ninevites. Hope will never expire for you if you hope in the Lord.