I need to add an inside margin (= the side where the book loses a little of the readable page to the gutter).
In the compiling setup, I see only “right margin” and “left margin”.
How can I add the inside margin? (= right margin to the left page and left margin to the right page)
This is only a guess I’m afraid - in Compile / Layout there is a check box that says “Do not add page padding to first document” whose default is unchecked.
I’ve not tried it, but my guess is that compiling automatically adds the kind of ‘page padding’ (inner) that you’re looking for?