A library, by definition, is full of flammable materials anyway :)

I remember this dude putting a bunch of materials to the test with a direct blowtorch flame. https://www.youtube.com/watch?v=8NC79e0oztM. I was hoping he had a demo of denim; it's mentioned in the video description, but skimming the video I couldn't locate it in a reasonable time. You can look at his demo on treated cellulose around 1:48

Denim Batt at 12:39

https://youtu.be/8NC79e0oztM?t=12m39s

bummer ><, I was hoping it would perform a lot better. The brief coverage of the poor performers explains why I couldn't skim it and land on it. Only the strong contenders had long enough coverage to skim with a mouse hover over the timeline and spot them in the thumbnail. Sounds like it was trading on low allergic response vs flammability according to his commentary.

I imagine it was sprayed with a flame retardant?

Fabrics used for insulation are treated with flame retardant.

People also insulate with cellulose -- ground up newspaper. It is treated with boron to make it flame retardant.