Has anyone found a function to do this? Nothing came up either in the Book script or ObjectReference script, or searching through the wiki. Or is this a SKSE thing.

Do you want to change the entire text, one paragraph, or a number of specific words?

Well I'll probably have a template like:

"Your account value is X, and you earned Y"

And change X and Y via a script.

The book or reference itself has variables and a script, so I could get the values from that?

I believe you would use an Alias here. It's how they do bounty letters. It says something like (I'm not familiar with the actual code, I have yet to look into that) "Hurry! All residents of , and kill the bandits before it's too late! They are hiding out in ." Or something. I'm sure you could easily do something similiar with variables so a book changes dynamically in realtime. (Each time it's opened, maybe not right as you're staring at it.)