The book was written in more or less in Chronological order except Chapter 17-21 in which two events was included which happened roughly at the beginning of the settlement in Canaan (at the for the second event) and they have nothing to do with any Judge at all. Given their odd existence, they are either as a strong message that the author wanted to give (may even be the main theme of the book) or they are later additions. I will address this in the final part of this article. Now, let me naturally divide the book into two and treat Chapter 17-21 as a kind of appendix. We will look at the main part of the book first.
As many people will probably have noticed, the main story line of the book is repeating of the cycle: people did evil, God punished handing them over to foreign powers, people repent and God sent deliverers (Judges to recuse them). However, one should notice that these are not equal cycle but in a trend of getting worse every time. I will come back to this later.
As some people would say, there are major Judges (those whose events were described in details) and minor Judges (those names just mentioned) as for major prophets and minor ones. As in the case of prophets, it may not be the case that minor Judges is inferior to major Judges but just that their events were not recorded in details. A natural question is why is there such a discrepancy in the records Judges. There can be several answers I can think of. The first is that the events of the minor Judges were not preserved in the oral tradition as good as the major ones (I am a bit doubt this since the author seem to know quite a bit about some Judges like Jair -- who seem to be quite important as he controlled thirty towns -- and Ibzan and Abdon). It can also be the case that in the eye of the author, the deliverances brought about by those minor Judges were not as great as the major ones and so their materials were ignored. A third reason, I can think of is that like some of the ancient Chinese historians, he wrote history not for sake of history but for making a point (i.e. those materials are carefully chosen to put up a picture).
Since there is nothing we can say about the first two assumptions, let us stick to the third and see what we can get if we assume this. As I mentioned about throughout the records of the Judges in chapter 3 to chapter 16, there is a tendency of going downside. Throughout the book, there are three public addresses of God to his people. The first one is in 2:1-3 when God told his people that since they didn't obey God (in driven all the people out and destroy all their altars), God will not do it for them. (2:20-22 seem to be a restatement of the first address of God.) The second one is in 6:8-10 when the Israelite did evil in the eye of the Lord (in worshipping other gods) and He again rebuked them. The third and final one is in chapter 10:11-14 when the Israelite once and again served other gods and God sent a very strong message to them this time warning them that God may not help them any more.
Another thing that we can notice is that for the earlier major Judges (Othniel, Ehud, Deborah/Barka and Gideon), the words: "and the land had peace for xx years" but not for the last two. It may due to the obviously reason that Jephthah only in power for six years and Samson virtually didn't delivery the Israelite at all. However, these two Judges came after the final message/ warning of God as stated above.
Now we turn to the major Judges themselves. We see that for the early Judges, they deliver the Isreality in a great way and the people followed them and celebrate with them in their victory and in the case of Gideon even wanted to hail him as king. But the tune for the later two major Judges were completely different. It is also interesting to compare the aftermath of the victory of Gideon and that of Japhthah when some trouble makers came to accused them. Gideon used good words to claim their resentment and deliver them in peace. On the other hand, Japhthah just killed them all.
Of course, there were already some bad things about Gideon being recorded that he destroyed Succoth and Peniel in revenge of their impoliteness to him. Moreover, his son Abimelech turn out to be a very wicked person. The record of Abimelech is very strange, he is not a Judge (at least not in the mind of the author because he said in 2:16, "Then the Lord raised up Judges..." and this mean that Judges, by his definition, is someone raised up by God). Another strange thing is his wickedness. This account is the first wicked account in this book Judges. His being having a place in this book is basically because he is the son of Gideon (and he did wicked things).
We can see the account of Abimelech seem to turn the overall atmosphere of the book seem to have changed. The two major Judges after him were both not good: Japhthah killed a lot of Israelite and Samson seemingly didn't fulfil his job as a Judge. Therefore, Gideon seem to be a departing point of good and bad or bad and very bad. We can see very clearly that the decaying sense from chapter 3 to 16 (and even more the case for the last two chapters).
So, the message to the Israelite is in fact very clear: if you don't repent, you will going downhill until you are perished (a typical Deuteronomic message) and don't think that you have heroes, general or even kings to save you, your only deliverance is from God. Given the tragic death of Samson as the end of this book, the message could not be any clearer and stronger.
Now, I would like to consider the final five chapters. Even though I consider myself traditional, I couldn't resist the temptation of thinking that these chapters are later additions (probably not by the original author). The author seemingly wanted to paint a picture that the early period of the settlement was a good period and he said in 2:7 "The people served the Lord throughout the life time of Josuha and of the elders who outlived he and who had seen all the great things the Lord had done for Israel. However, we find that wickedness of the Benjamites tribe happened during the time of Phinehas -- a co-leader of Josuha. On the other hand, it said at least twice in these chapters that such evil and chaotic things happened because there was no king in Israel. However, we see the account of Abimelech that he certainly was a sort of kingly figure (and was actually a local king) and we see his reign brought even more chaotic and evil. So, again, this seems another contradiction.
I couldn't think of any reason of adding this appendix. The only thing that I find them fit in to the text is the general chaotic tune that fitted the atmosphere being painted in chapter 9-6. Maybe, it is just that some people find some historical materials that they think fitted into the general frame of the book of Judges and so it was added there. Or is it, as some people say, two accounts that added to support the need of a king.