When the updated version of Chapter 7 of the django boook is released
online, if the same description persists, it would be worth making a
comment over there. Although the book is written by two of the primary
authors of Django and being updated by one of them (Adrian), this list
isn't the place to report bugs, since we have nothing to do with the
content of that book.
Basically, though, any relative URL is going to relative to the portion
of the URL prior to (and including) the final slash. It behaves
identically to relative paths in filesystems in that respect. That's one
of the reasons why ending most URLs that don't request a particular
static file with a trailing slash isn't a bad idea. It makes relative
URLs a little easier to control: you don't have to know the name of the
current page to submit a form back to itself.
Regards,
Malcolm