I want to create a book that is like a record book, it will store the number of times the player has done certain things, bit like the Achievements tab in the players journal.

So I would need a way to edit the text of the book each time something happens. Is this possible?

with OBSE string functions, yes.

Is it simple to script? I'd say no. I've never done it before, but someone lese here should be able ot help you write a script for it.