The cockpit voice recording.
There have been numerous posts about the psychology involved. Learning from this is vital because at face value the event seems bewildering.
One issue is about denial. #322 Portmantau's post included:
3. AC 759 believes what ATC tells him and prepares to land.
I suggested in an early post that the PNF doesn't quite believe what he's being told. The question mark hanging over his reply is clearly discernible.
I think it's important to know if the part of his brain processing that vital issue emphasised the point with the PF. Verbal exchange between pilots the moment after that call is of the utmost importance. So yes, the voice recording couldn't be more important.