Master Dungeon Guide

The Master Dungeon Guide is an item you carry around in your inventory. Completing dungeons will add an entry to the Guide. It is unknown what the reward is for turning in a completed Guide to Tyr the Skaald (Norn), Kodan (Deldrimor), or Gedrel of Ascalon (Ebon Vanguard).
 * Note, completing dungeons without the Guide in your inventory (e.g. during the sneak peek weekend) will not add them to the Guide.

Trivia
This may possibly be a reference to the Dungeon Master's Guide of Dungeons & Dragons fame, from which Eye of the North has apparently borrowed somewhat. As well as having a similar name, the primary colour of the book icon is close to that of the real book.