OK, its really simple man.
Open up your TES editor, go into Items>Book
From there you can either choose to create a new book by right clicking anywhere in the Book section and clicking New, or like me, you can just duplicate an old one by clicking edit on any previously existing book, changing the ID to something new, then clicking Yes when it prompts you if you want to make a new form when you hit OK down the bottom. Dont click no or you'll delete the other book

Now, to edit the actual text, i think it uses HTML, but im not sure. Just look at some of the other books in the editor and youll get a pretty good idea on whats what.
Thats it, easy as. I hope this helps you.