These are the two books in the Bible named after women. There’s an obvious message here (whether or not the titles were themselves inspired by God): Not only men are worthy in God’s eyes. The eponymous women are admirable in many ways, including their being bold and brave and smart, but it’s interesting that both work within a patriarchal system that is accepted and in no way disparaged.
An obvious lesson that Ruth teaches is the importance of loyalty — including a willingness to put others ahead of ourselves — and faith through the worst of times. And it shows God’s willingness to tap unlikely people to play important roles, both in other people’s lives and in fulfilling His larger plans — “unlikely” here being someone poor, non-Jewish, and female. Ruth was also brave, loving, and hardworking, and it seems fair to conclude that we are shown those traits so that we will admire and emulate them.
Likewise, Boaz is generous and compassionate, and no doubt we are to learn that these are traits also to be embraced ourselves. It is interesting that there is in this story no shame in being poor (like Ruth) — but there’s nothing wrong with being rich (like Boaz) either.
That seems to me, by the way, typical of the Bible’s attitude toward money: There’s nothing wrong with having it, but it’s also no disgrace not to have it; we’re not to be greedy, but covetousness and envy are sins, too; keep the value (and potential pitfalls) of money in perspective.
Esther teaches that we should be willing to take big risks — to lay our lives on the line for God. It teaches also, as does Ruth, that God works in surprising ways. In particular, Esther herself appears to have been rather secularized (her uncle Mordecai not so much), but God still asked her to step up to the plate and she did.
Mordecai’s refusal to bow to Haman puts the whole Jewish people in jeopardy, but there is no hint that Mordecai was wrong to take this highly principled stand. This is very much an Old Testament story: ancient blood feuds, no forgiving, inevitable slaughter one way or the other, and so forth.
I’ll note that Xerxes is really not someone to look up to in this book: not a fundamentally bad sort, but impulsive and manipulable; I’ll also note that this book take place around the time of the battle at Thermopylae.
Finally, for both Esther and Mordecai, compare their royal relationship with Joseph’s rise under pharaoh, and with Daniel’s relationship with royalty as well, for that matter. It’s perfectly fine to be a good staffer, to play second fiddle well. (Well, Esther was not really a staffer, but you know what I mean.) In all these cases the head guy was a Gentile, too; working and living with Gentiles was fine, but you had to keep your Jewish identity and act accordingly, a prescient lesson for the Bible to have taught Jews, since that would be their fate in most cases for the next 2500 years.