I haven't had a chance to read this, just scanned it briefly - the author would be correct in noting that authorship never served, at least for the Reformers, as the basis of canonicity. They always proceeded from theological truths to reason and empirical evidences and never reversed that process, which is common position among textual criticism today.
The author states this:
"The content of our fourth Gospel is true and trustworthy. This is not what is in question. Nothing presented herein casts any doubt whatsoever on the legitimacy of this Gospel as inspired scripture, nor its rightful inclusion in the New Testament. No one should think that this study in any way challenges the words of God’s inspired writers or the accuracy of scripture. Any such innuendo would plainly be a flagrant distortion of what is discussed herein."
Not having read his work, and just presuming that his intentions are honorable, then questioning the authorship of John does not in and of itself cause any problems. I don't know that I'm going to take the time to read this as it doesn't intrigue me that much - but it is interesting.
But you might try and not overreact if you haven't read it, I just don't see how this is a perceived as a threat to some.
Member, Covenant Reformed Presbyterian Church