"Woe to the bloody city, all full of lies and plunder- no end to the prey!" - Nahum 3:1
This is the message that Jonah wanted to preach. Instead he got to preach a message of repentance to the bloody city which was full of lies and plunder.
From man's raw perspective (and in these cases, the perspective of Israel in particular) the message of Jonah was good news for the Ninevites and bad news for Israel. In contrast, the book of Nahum was good news (finally) for Israel and bad news for the Ninevites. There are a couple of applications that we can make.
First, God is sovereign and his timing is not ours. When we speak of his sovereignty we mean that because he is outside of time and sees all, there is no escaping his judgment. This also brings to mind a New Testament truth: that God is long-suffering, not willing that any should perish, but that all should come to repentance. We know from Jonah that the Ninevites did repent, to the praise of His glorious grace! From Nahum we learn that he will ultimately bring justice about, but that it may not be in our time or generation. Jonah was long-gone, and all the people that were alive in his day, both in Israel AND in Nineveh. So, don't despair thinking that God doesn't notice - He does, and he will not be mocked. His timing is different than ours.
Secondly, let's look at two messengers. Both Jonah and Nahum are prophets, called to deliver God's message. They bear different messages, but more importantly, it is not their right or privilege to change the message. Jonah fought it at first, unsuccessfully, and ultimately delivered the message that God called him to, albeit tersely. The application is this: we are called to deliver God's message, no matter how we personally feel about it or the recipients. This is a sword that cuts two ways. On the one hand, we must give the good news of God's offer of salvation from sin to all without exception. It is not for us to decide who should and should not hear this message. On the other hand, we also cannot change the message. Just as Jonah preached repentance, so Nahum preached wrath and destruction. While we are not Israel or preaching to specific cities or nations, part of the message we preach is that God will ultimately judge the wicked and there is a day coming in which he will pour out his wrath against the wicked. Therefore, let us share the entire message that God has given us, for through it He has ordained to bring salvation to all who are His!