The textual form is what form it is in e.g poem, book, website, song etc. Different forms have different consequences to the reader, e.g a website can be updated all the time, whereas a book will always be in that one form, only able to give that one viewpoint. You would talk about how the Fiftieth Gate, being in the form of a book, has made the story more personal, the r/ship between history and memory more evident etc. You could possibly talk about the artwork on the front (if any) and also say how that adds to the r/ship. This is all just what I think btw, and how I would interpret it!