I went through the Warrens in Necavrillir and found the book. Then I went into the other one, defeated fire, water, life and.....sand, plus Ulik. I tried to leave but it told me I'd lose everything. How am I supposed to get back out to the lobby to go into the Hall of Valor?
After you fight all those "little" bosses, there is a "big" Death boss at the top of the ramp. He's one of Jotun's brothers. Unless you planned to fight Jotun (the main boss in the middle dungeon), fighting Ulik doesn't matter.
But it's true, leaving before you defeat the entire boss will wipe out the progress you made. If all you were after was the book or yardbird, that's no big deal. You'll still have the credit for those.