Someone explained me the trick for the quests I was blocked : you can use the action key even if there's no special icon.
So, for the book one, I had to use F on some books till I got it.
For the Melechar one, it's just in the village next to our city, this chest :


For the dwarf god symbol, you have to hit some block near the stone hand just next to the dungeon portal (don't hit the portal) :


For the Chamber of fire quest, just hit the altar in the fire elementals room.

So, if you get other quests of this kind, you should be able to complete them easily now.

[Linked Image from w3.the-kgb.com][Linked Image from w3.the-kgb.com]