The Book of Nachum begins with a Prophecy concerning Nineveh, the capital city of "Ashur," the Kingdom of Assyria. Note that this is the second Prophecy concerning this city, but how different!
In the first, Yonah ben Amitai had been commanded to prophecy to Nineveh, which had sunken to such a degraded state that they were comparable to Sodom, "In forty days, Nineveh will be overthrown." But then the King of Nineveh had taken Yonah's words to heart and had been the center of a movement of "Teshuvah," of Repentance - one of the rare times in the Bible that we see Repentance among the nations. And indeed Nineveh was overthrown - from bad to good!
But now things are radically different. Nineveh has fallen again, this time displaying more evil than before. And Hashem is informing them here, through the Prophet Nachum, whose name means, paradoxically, "comfort," that this time they are doomed, doomed to destruction at the hands of Nevuchadnezzar and the Babylonians.
Nachum has a distinctive "literary" style, in a number of ways. He uses repetition in an interesting and unusual manner, as in this somewhat frightening verse (Nachum 1:2):
"E-l Kano Ve'Nokem HaShem,
"The L-rd is a jealous and avenging G-d,
RASHI explains that the three occurrences of the word with the root of "revenge" corresponds to the three exiles that Sancheriv inflicted upon Israel.
He also uses alliteration (use of consonants to achieve a literary effect) and onomatopoeia (definition in American Heritage Dictionary: the formation or use of words such as "buzz" that imitate the sounds associated with the objects or actions they refer to).
An example of the former is in Nachum (2:11) where the Prophet describes Nineveh as "Bookah, U'Mevookah U'Mevoolagah," "kah," "kah," "gah," "empty and void and waste;" - the double o's in English anyway, and the sound of the double o (the equivalent in Hebrew is the "shuruk" or the "kubutz"), also suggest emptiness.
Of the latter is the description of the sound of chariots in Nachum (2:5) as "Yishtakshekun," "they jostle against one another."
Though Nineveh has powerful chariots (Nachum 2:5) and attacks and destroys its enemies like lions (Nachum 2:12-13), HaShem says,
"I am against you, says the L-rd of Hosts,
"Woe to the bloody city
" wherein is heard
"Behold I am against you,
With all of its might, Ashur was not able to stand against the "Destroying Messenger of HaShem," Nevuchadnezzar.
"And it shall come to pass,
"Your shepherds are in deep sleep,
And, unlike the Jewish People who will also be scattered, for you
"There is none to gather you up."
According to the RAMBAM, Nachum was a link in the Chain of "Mesorah," and received the Tradition of Torah from Yoel and his court.