The FF games would make horrible books for the following reasons:

1) The stories aren't really all that good. These are stories written by programmers and producers. Even the most convoluted FF plot is easily outdone by Siddartha. That's another problem, FF's answer to making a plot better seems to be "make it more incoherent". This doesn't necessarily make for a good read. If the incoherence is resolved by some sort of lengthy explanation that places all the pieces of the puzzle together, that's not so bad, but it makes you wonder why you were forced to puzzle over it for so long. The writers also have no concept of things like subtlety, wit, satire or florid language. It's a constant bombardment of overwrought caracature of the feelings that the writers are actually trying to express. This is fine for things like video games where stories aren't really the prime focus, but for books where it's the ONLY focus, it's really not so good.

2) Does the average FF fanboy REALLY have the attention span required for a book that's as long as FFVII would have to be?

If anything, a graphic novel would be good, but not books. FF games just wouldn't work as books.