The book of Nahum records the messages of the prophet Nahum. As with most prophets, Nahum’s message focuses on current events, but in this case for the people of Assyria.
The capital of Assyria was Nineveh, so we can read the prelude to this book in the book of Jonah.
Nahum, like Amos, addresses the faults of a foreign nation, Assyria. Assyria, was long an oppressor of Israel and Nahum predicts its destruction. There is no call to repentance — as is often seen with other prophets — just judgment.
From a literary standpoint, the book concludes with a masterful ode to the fall of Assyria (Nineveh).
Nahum is sometimes called a minor prophet. This doesn’t mean he wasn’t important, but merely that the book named after him is shorter. (Compare this to the major prophets, whose books are much longer.)