I am not a copyright lawyer, however, to use any illustration for the intent of publication without the originator's express consent is *definitely* a copyright infringement.
Permission for the use of illustrations would have to be given by the painter himself (Vargo) in order to appear in a book of any sort. I don't recommend violating others' copyrights ever ever (of course I don't... it's artistic embezzlement, and it sucks).
"Books" written by an individual *for individual use* (i.e. a collection of writings on the Gothic deck), but not containing illustrations would not violate copyright. In other words, if someone felt inspired to write a book about their use of the Gothic deck, that would not be a copyright infringement.
There would definitely be some gray area if someone wrote a book about using the Gothic, and then found a publisher (i.e. they stand to make an income) for the piece. I really don't know if that's a copyright infringement or not. Better safe than sorry, is my advice. Ask permission, always, when it comes to borrowing artistic materials (songs, pictures, text, etc).