ISTANBUL, Turkey — In the hushed reading room of the Turkish-German Bookstore, you’d hardly know Turkey and Germany are seething at each other.

“We are like a family here,” said Thomas Mühlbauer, the German-Austrian manager and co-owner, with his brother, of the store on Istanbul’s main shopping avenue, as students peacefully pored over their books.

The depth and history of the relations between Turkey and Germany mean that much stands to be lost in any potential rupture. That is what has made the precipitous chill between them all the more surprising.

After weeks of acrimonious sniping, the question forcing itself on both sides seems to be just how far can things deteriorate before hard consequences for all become unavoidable.