There are official artbooks from the creators of the game. These are not manga. Manga are the equivilant of comic books, they tell a story. Art books are just collections of art. There are many many official art books with Amano/Nomura art in them, but there are NO official manga. All manga for the Final Fantasy series is done by fans, usually in professional companies or 'circles'. Some look official as hell, but they arent.

Most everything in Japan has these fan-comincs (doujinshi) made from them. Every manga (Sailormoon has hundreds of spin-off doujinshi), anime, video game, and even popular actors and singers (Gackt has quite a few doujinshi centered on him) have their own. But these arent official, and are due to the loose copyright laws in America. They are no different from the fan art and fan fiction we see all over the internet. They arent official.