Sales differ significantly across BookBub categories. But how much is that variation due to simple differences in subscriber counts vs. discrepancies in reader engagement?
We can start to answer this question by examining how average purchase rates — or the percentage of BookBub subscribers in a given genre who buy a title — vary by category. A higher purchase rate means that readers are generally more engaged, while a lower one suggests that subscribers in that category might buy fewer books on a per reader basis, even if the raw sales numbers are higher.
What we’ve found is that while BookBub readers are active across all our lists, average purchase rates vary quite a bit. In particular, many of our smaller categories — for example, LGBT and African-American Interest — have extremely engaged subscribers. This suggests that while these genres might attract a smaller base of total users, they also tend to be more targeted lists and consequently draw more active readers.
On the other hand, many of our largest categories have lower average engagement rates. This makes sense — in addition to fervent fans, there are likely many more casual readers of a genre like mysteries than there are of more niche categories. An even more extreme example of this is our Bestsellers list; people might broadly like bestselling books, but it’s more difficult for any one title to appeal to everyone’s interests. On the opposite side of the spectrum, many of our romance genres have been subdivided into more targeted categories, raising activity levels on a per subscriber basis.
Other factors associated with list size, such as how long the category has existed on our site and how many books we feature in it each day, can contribute to engagement as well. New BookBub users in particular tend to be the most active, helping many of our more recent category additions benefit from higher average purchase rates.
Many of you probably won’t be surprised to hear that Erotic Romance seems to buck all trends, being one of our older, larger categories and consistently one of our most engaged!
To continue our investigation, we also looked at the correlation between click-through rates and conversion rates. For example, it could be that readers in some categories click on fewer books in their daily email, but are generally more likely to buy a title once they reach its product page. Or the two trends could go hand in hand — less engaged subscribers might also be less likely to buy once they click on a book.
What we found was that many of the genres with lower overall activity actually have very high conversion rates. Mysteries, for instance, had one of the best averages, even though its purchase rate was on the lower end of the spectrum. The chart below, which compares each category’s average CTR with its average conversion rate, illustrates the lack of correlation between these two metrics: 
Regardless of their relative performance, there are a large number of very enthusiastic readers in every BookBub category, and most genres with slightly lower purchase rates still drive thousands of sales for the average promotion. But along with data on things like subscriber counts and raw sales, it illustrates that no two BookBub genres are exactly alike, and that list size is far from the only determinant of reader behavior.
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