Now you have to return to Topple — you probably remember the way back, and if not the path is pretty linear, and you can't get to Wendel at the moment. The boy's strengths can be pretty useful here as he does double the damage you have and has almost twice the health to survive the more tougher monsters.
Once in Topple the mysterious boy will head off. Return the book to the boy in the south-eastern house and then speak with the grandfather — he will give you an Escape Rope which will pull you out of whatever dungeon you're in to the last save point you used. The grandfather will also offer to let you rest up at their place. Just agree and leave.
Save your game at the golden Mana Goddess in the village, then enter the north-western house and talk to the guy inside. He mentions something about the young man that had recently helped you out asking about Sir Bogard…Therefore, you must rush back to Cascade Cottage!
Save, and head off to Cascade Cottage.