I find this question somehow similar to this one: What should a rational person accept as a miracle? In such a way, I do believe that the answer goes pretty much in the same way: it all depends on what you define by "rational" and what you define by "God" (although... do'h, that's totally breaking your question apart).
The point is that if a rational person would not accept God-inspired-books because it's just not "rational" to accept or because there are no direct proves for that, then you're defining being rational by being proof-based. Then, any proof of non-human inspiration could be the proof for God-inspired (but this may not be not actually correct) and the source of an inspiration can be definitely subjective. Pretty much in the same way as what do we consider a miracle and what do we consider extra-human-inspired, it is so subjectively accepted that there's no easy "rule" to define what's into it and what's not.