As you progress through the God of War Ragnarok you’ll encounter various favors and The Elven Sanctum in the Alfheim region is one of them. After you complete this you will be able to unlock the Consul's Journal. This item is needed for a side quest, but after you collect it, you keep it in your inventory.
If you are wondering whether to sell it or keep, well, the answer depends on your interest in this item as it’s no longer needed after bringing it back to complete the quest. Therefore, you can sell it without having an impact afterwards, but you will however be entitled to some additional dialogues.