If you can afford it buy Mikes Master Library, you probably will not understand a lot at first but it will help you visualize and understand a lot of the code.
The only other advice I can give you is to go through and understand the definitions in article 100 thoroughly as well as any definitions within the articles themselves. Understanding definitions helps you navigate the code book to the correct article. For example, when I was an apprentice a JW wanted to make a splice in a panel but he didn't know if it was code compliant. So we took the code book out and went to article 408 which cover panelboards and found nothing about splices. The reason we didn't find anything is because we didn't know our definitions. The term panel is a generic industry term but in order to navigate the code book you can't be generic.
If we would have looked at the definition of panelboard in article 100 we would have discovered what a panelboard is and with this information, we would have gone to article 312 to find our answer.
