Answer:
In the sentence "I've heard of the author, although I haven't read his books," the subordinating conjunction is "although".
Step-by-step explanation:
A conjunction is a word used to connect other words, phrases or clauses in a sentence. A subordinating conjunction is used, more specifically, to connect an independent clause to a dependent one, establishing a relationship between them. In this case, "although" establishes a contrasting relationship, meaning that the clauses present ideas that somewhat oppose each other.
In the sentence "I've heard of the author, although I haven't read his books," the contrast is in the fact that the speaker has never read any author's books. The speaker has heard of the author, which could lead the listener into thinking he has read something by that author. But, in contrast to what could be thought, he hasn't.