The effectiveness of the two sentences can be evaluated based on the clarity of the information they convey.
The first sentence provides a broad context about the number of books purchased, highlighting the scale of the market. The second sentence introduces a contrasting idea about authors receiving only a small fraction of the profits, which adds depth and interest to the topic.
Given that the second sentence contrasts effectively with the first and provides valuable information about authors' earnings, one could argue that the second sentence is more effective.
So, the answer would be: The second is more effective.