Help CenterDaily Double Books is a feature that allows members to double the token value of their book. It was a great idea to help people get extra reviews quickly, but unfortunately, it caused a significant imbalance in the ratio of available books to available reviewers.
The feature still exists today, but credits are no longer available for public purchase.
Instead of the Daily Double promotional feature, you can use the Bonus Book feature, which provides the same promotional value as a Daily Double book. You could even bonus more than double the token value if you wanted to take things to the next level.