I’m confused about the principle that if the muḍāf in an iḍāfah construction is singular, it indicates generality. In a simple textbook example, does kitābu ṭ-ṭālibi mean all of the student’s books? What about kitābu Allāhi? Is there a difference between fuṣḥā grammar and the sharʿī grammar?

There is no difference between the grammar points because this is linguistic. However, I went through this in detail—I believe I expanded on this in Level 2 and this is specifically mentioned in the book, "القواعد الحسان في تفسير القرآن." It requires a longer explanation that is better handled in one of two ways: 1. Discuss it during the Friday live Q&A. 2. Go back to those specific lessons to review what we covered there.