Text: To define the two terms below, I need to establish that when I say 'text' I am only referring to the written content of a book (words), and not the entire book as an artefact (content and form).
Para-textual: Elements outside the text, 'packaging': jacket (book cover), blurb, prelim pages (imprint page, title page, dedication, etc), folios, running heads/feet. Chapter head illustrations? Picture sections? Elements added after the author has written the text; material generally generated by the publisher/editor/designer?
Inter-textual: Non-verbal elements within the text, supplementary (?) information: photographs, diagrams, illustrations that are integrated into the text. Conceived or generated during the composition of the text (or can they be added afterwards where necessary?)
The creative role of the book designer (where the designer has creative agency) generally involves para-textual elements (book cover design, typesetting design). If a designer is involved in generating inter-textual elements, these are usually producing illustrations/diagrams from a fairly strict brief (what I would call rendering rather than creating). In fiction with integrated (inter-textual) graphic elements, does the designer or writer maintain creative agency?