I want to find the correct way to say that I´m thinking and documenting about all the details of my book, possibly making some diagrams, describing characters, etc., instead of actually writing it. Is there a verb for that? How would I say it?
If you document these, you would call that an "outline", a "sketch", or a "summary" of the book. If you're just thinking then you would call that an "idea" for a book, as in:
I have a great idea for a book, let me tell you about it!
You can also have ideas for any kind of creative work: movies, artwork, music, etc. -- until you start on it, after which you might call it a "work in progress".