What would a ‘missing’ book do?
What would be it’s effect?
1. Reinforce what we alREADY know, or...
2. Reveal to us that OMG!!!! We ain’t saved after all!!!!
Well, of course, we can’t know exactly what such a hypothetical thing would do, because we can’t read it. If it was an authentic work, then it should be in line with the rest of the Bible, though it might add some new insight that doesn’t contradict with the rest.