Are you reffering to those 2 doors at the lower lvl?
I had an interesting expiriance with them too, when I first got in the Nidavellir, I had access to all 5 doors, now that I started with front/left and fornt/middle the back doors are no more accessible.
I still have to do Jotun in the Front/right, I was hoping they will be accessible again after I that.
Daniel Mistrider Magus Sorcerer, level 38 Balance & Storm
Although I know many wizards below level 40 have had the ability to get to Nidavellir, it is supposed to be for level 40+. You aren't supposed to have access to that area until you reach that level. Once you do, you get the king's quest (can't remember the name) and sent to fight Jotun and his brothers. It is best to fight his brothers first. They reside on each side of Jotun (door in the middle). If you defeat them first, it is easier to beat Jotun. Just be aware that there are many traps in those dungeons. Also, the book is in one and the bird is in the other.