I assume the same is for books, but not sure how the process is maintained. For the newspapers, they stored them in a climate controled room for several weeks to get the moisture level down to exactly the level they wanted, I don't remember the exact percent. They treated them with a chemical that arrested the decomposition from UV light and laid them flat for quite a while, about a month without being disturbed, to get rid of some of the creases and wrinkles that could be fixed. The last step was to seal them in a plastic looking case that allowed the spread pages to be seen. The cases have small "valves" (I don't know what they are actually called) that damper changes and help them monitor moisture and whatnot. Presumeably, if they have to be re-done, the case can be unsealed but the papers left inside while they restore proper humidity and stuff.

I do know that it cost significantly more than I'd have been willing to pay out of my own pocket. I was happy to let them take custody and preserve them. My pockets aren't that deep.