Is this correct One of the best things about reading short stories are that you can complete them in one sitting?

It is not quite correct. The correct sentence would be, " One of the best things about reading short stories is that you can complete them in one sitting." What you wrote is incorrect because you chose to say 'One of the best things'. One is not plural, and goes with 'is', whereas plural (such as 'Some of the best things') go with 'are'.