The effectiveness of the two sentences can be analyzed in terms of the clarity and impact of their messages.
The first sentence states a clear fact about the number of books purchased, while the second sentence provides insight into the profit distribution for authors.
Without further context about what is meant by "effectiveness," it seems that both sentences convey important pieces of information related to the book market and authors' earnings. Therefore, one could argue that they serve different purposes.
However, if we consider which statement has a stronger impact regarding the authors' situation, the second sentence might be seen as more effective as it highlights the disparity in profit collection.
Given this analysis, I would choose:
b The second is more effective.