We lost alot of our XVI-XIX century history (foundation of Asunción, treaties with the natives, comunero's revolt, Jesuit and Franciscan missions, early independence, Francia's Era, Carlos Antonio Lopez Era, Blueprints of early Asunción, etc etc) when our capital city was occupied by the Brazilians in 1969. Most of the literature captured was burned and whatever was of interest to the Empire was sent to Brazil as war treasures by a Brazilian diplomats, who gifted it to his relatives or the wealthy artistocracy. So most of our "detailed" history is, unfortunatelly lost forever. Paraguayan Diplomats are still trying to recover it, but being realistic, its a lost cause.

But there is still lots of books written by historians that compiled and made researches of that time period. If you understand Spanish, you can find books of Fabian Chamorro, he usually writes about those periods. I highly recomend it. He also made a show with another guy visiting sites of the war, where he talks about this infamous war with great detail. The show is called "que pasó acá?", it is a very unbiased interpretation (eventhough its the Paraguayan side of the story) of the war.

Regarding Guaraní, well that is a complex story. We can attribute the widespread of the language to the following factors:

-The Jesuits. They fell in love with the language of the Guraní and transcribed it. Giving it a written form was fundamental.

-Dr. Francia. He is the father of the National Identity. Eventhough I wouldn't apply his policies nowadays (closed the borders, not allowing people to go, and if they got in they could never leave, closed commerce, etc.) Thanks to those extreme measures, Paraguay didn't fall to the sphere of influence of bot the United Provinces or Brazil.

-The Glorious Paraguayan women. Those women rebuilt the country after the war, and kept the Paraguayan culture alive.