What are some interesting facts in the book The Underdogs by Mariano Azuela?

Although this is a novel, Azuela, working as a medic, was an eye-witness to the Mexican revolution, so his work gives us a bird's-eye view of the times.
In the book, real historical characters make an appearance, such as Pancho Villa. Villa was a famous general in the revolutionary army and Azuela actually served under him. Another historical character, Francisco Modero, who was assassinated but had earlier overthrown the repressive former regime to become President of Mexico, is invoked. The reflections on Modero reveal some of the cynicism expressed by the common people, who felt that once they had helped Modero achieve power, they were turned on and told to go back to work, "half naked and hungry," just as they had been before.  
Most interestingly, the novel offers facts about the social history of Mexico in this time period. We witness the primitive conditions in which the peasants lived: in the opening scene we find a room lit by a tallow candle and a children lying on a bed, covered in rags.
We learn too that not only today are people concerned about a surveillance state. The passage below, where the policeman has his ear to the door, indicates that the same issue bothered people more than a century ago and gives us a colorful picture of what life was like in Mexico at that time:

I was born in Limon, close by Moyahua, right in the heart of the Juchipila canyon. I had my house and my cows and a patch of land, see: I had everything I wanted. Well, I suppose you know how we farmers make a habit of going over to town every week to hear Mass and the sermon and then to market to buy our onions and tomatoes and in general everything they want us to buy at the ranch. Then you pick up some friends and go to Primitivo Lopez' saloon for a bit of a drink before dinner; well, you sit there drinking and you've got to be sociable, so you drink more than you should and the liquor goes to your head and you laugh and you're damned happy and if you feel like it, you sing and shout and kick up a bit of a row. That's quite all right, anyhow, for we're not doing anyone any harm. But soon they start bothering you and the policeman walks up and down and stops occasionally, with his ear to the door. 

Towards the end of the novel, the modernity of the early twentieth century collides with the world of the peasants when government forces use machine guns to great effect: "They mowed us down like wheat,"  reports Solis. Finally, the novel is unflinching in its assessment of the failed dreams of the revolution.  As Solis says:

What a colossal failure we would make of it, friend, if we, who offer our enthusiasm and lives to crush a wretched tyrant, became the builders of a monstrous edifice holding one hundred or two hundred thousand monsters of exactly the same sort. People without ideals! A tyrant folk! Vain bloodshed!

In fact, the lower classes in Mexico remain poor to this day.