Amazon released some strong revenue numbers Thursday around KDP Select, its lending library program for self-published authors.

The data, released in effort to ward off uncertainty about the program, suggest that KDP Select is generating some serious revenue for some bestsellers, but offer few details about its benefits for the vast majority of participants.

KDP Select is a lending program that encourages self-published authors and publishers to make their work available exclusively in the Kindle Store for 90-day periods. It also makes their works freely available to borrow by Kindle users for at least five of those 90 days. Those who agree to both terms are eligible to take part in a $6 million annual royalty fund, which pays dividends based on each author's share of total borrows during that month. An author only earns revenue if one of his or her books is borrowed.

As Laura Hazard Owen of PaidContent pointed out at the time of the program's launch, KDP Select offers some obvious benefits — namely, increased exposure — but also has some significant drawbacks. The exclusivity clause prevents many would-be lenders from selling their books through other vendors with self-publishing programs, such as Barnes & Noble and Smashwords. There's also no guarantee will make much, if any, money from the program.

In the statement sent out Thursday, Amazon disclosed that there are now more than 75,000 books in the Kindle Lending Library. Customers, who must possess both a Kindle device and Amazon Prime membership to borrow a book, borrowed books 295,000 times in December. Authors earned $1.70 per borrow from a $500,000 fund. The top 10 authors earned an average of $7,000 each in royalties.

Most promising was the sales boost many authors received as a consequence of adding their books to KDP Select. The top 10 authors in the program saw a 30% increase in royalties received from sales of their books over the period. Including their earnings from the loan fund, their earnings from Amazon were up 449% on average from November.

One of the top 10 earners, paranormal romance writer Carolyn McCray, said that participating in KDP Select quadrupled her royalties. She earned $8,250 from the lending program alone plus, presumably, a bonus in paid sales. "To say the trade-off of exclusivity on Amazon for the Kindle Owners’ Lending Library has been a profitable one would be a gross understatement," she gushed.

Those are strong numbers, but they leave many questions. Just how many of those 75,000 books available to borrow are from self-published authors? Books were borrowed 295,000 times, but how many individual books were borrowed? How many were borrowed once, or not at all? We know the top 10 authors made some serious cash, but what was the median amount made? How many authors made nothing?

Amazon could not be reached for comment.