If you want to build a "Bible", you will be forced to go through a training organization in my opinion because people who have recovered an NX training manual have a non-disclosure clause in the training agreement.
I don't think that Visiativ does NX training, so I recommend FEALINX (Ex CADESIS). I was trained by this company and they are very knowledgeable and were the only ones to be certified by the car manufacturers at the time. Maybe that has changed