Your opinion in this instance is the joke. Fan fiction? How arrogant can you be to assume that merely because you haven't seen it it isn't real?

You don't get to decide what does or doesn't count.

Most shows have supplimentary material that detail their universe outside the show. Does that mean the DC Fringe tie-in comics aren't real? That Doctor Who Audio dramas aren't cannon? The shows runners and creators created these 100% OFFICIAL side stories. You don't deem whats cannon or not. If that's the opinion your going to have then take it elsewhere. It adds nothing constructive to this topic.

So this story was written by Amemiya?

Kobayashi Yasuko wrote Zero: Dark Blood and will be writing (probably done by now) Garo anime.

Garo: Flower of Makai, and Ryuga's movie are unknown (but 80% chances they will be written by Taguchi Kei)

Regardless of who wrote them, if they are authorized by the company and creators which own and produce the show it doesn't matter. That still makes them cannon or official. If it was unsanctioned, sure, it'd be fan fic or whatever. But totally licensed approved and marketed materials are far far from it.

While Amemiya "created" Garo, it is Taguchi Kei's ideas building up the entire Garo world.
He is the brain behind all seasons, movies, and novels.
The book I mentioned earlier is also his work.

The GARO Ankoku Makai Kishi Hen novel and Yoseki no Wana novel were written by Yuji Kobayashi, one of the main writers of season 1 and both were approved by Keita Amemuya. Seriously, how moronic can get you get? Fanfic because it's not happening on screen? Oh wow, I guess you really never heard of the many many Japanese media franchise that have novel and supplementary tie ins which are official? Just because the "main creator" doesn't write doesn't mean it's not official or canon, many of the idea still comes from the main creator but a lot of time they don't have the time write to all of them and because workload that would have been involved, they usually leave it to writer with their approval. Using novels and supplementary materials to fill in the hole in the main series happens a lot in Japanese media franchise and AFAIK, other media franchises as well.
So, basically unless you saw them on screen, you're not gonna considered it canon even if the main creator does? Oh okay, then why don't you just live inside your own little world called "Dr Kain's magnificent world where everything has to have my approval to be official"

At the end of the trailer one of the eye on the Garo armor is damaged.

Is this novel even worth all of this conflict over? I mean not everything canon with Garo was that great. Kiba Gaiden for instance, with the exclusion of the fights, was rather boring and my Garo experience was not improved having watched it.