Synopsis: The concept behind Lettres persan is a Persian visiting France and writing letters to back home about what he encounters, which is basically just a vehicle for Montesquieu to ridicule society, government, Christianity, and the French.
In De l'Esprit des lois, as the title suggests, Montesquieu describes the nature of law and the reason and necessity behind it. He then describes three major types of government: republic, monarchy, and despotism.
Response: The Lettres persan were very enjoyable to read since they were so chock full of satire. They reminded me of C.S. Lewis' The Screwtape Letters, (I highly recommend it if you haven't read it!) because of the format and the critical content. I wonder if Lewis was inspired by Lettres persan.
I found De l'Esprit des lois fascinating in that it was written from the perspective of one who had only ever lived in a monarchy. Living in the US, we're taught from birth that democracy is the only way to go and that all other forms of government are inferior or outdated or something, but Montesquieu finds fault with all major forms of government and I think he's completely right. Don't get me wrong, I'd definitely rather be living in a republic than in a tyranny, but if the politicians and public servants running the republic are corrupt then it's no better than other types of government. I was also impressed that Montesquieu had such insight into the nature of law and into different forms of government without having experienced them firsthand.