Here’s a riddle: How do you make your book a best seller on the Kindle?

Answer: Give copies away.

That’s right. More than half of the “best-selling” e-books on the Kindle, Amazon.com’s e-reader, are available at no charge.

Although some of the titles are digital versions of books in the public domain  like Jane Austen’s “Pride and Prejudice”  many are by authors still trying to make a living from their work.

Earlier this week, for example, the No. 1 and 2 spots on Kindle’s best-seller list were taken by “Cape Refuge” and “Southern Storm,” both novels by Terri Blackstock, a writer of Christian thrillers. The Kindle price: $0. Until the end of the month, Ms. Blackstock’s publisher, Zondervan, a division of HarperCollins Publishers, is offering readers the opportunity to download the books free to the Kindle or to the Kindle apps on their iPhone or in Windows.

Publishers including Harlequin, Random House and Scholastic are offering free versions of digital books to Amazon, Barnes & Noble and other e-retailers, as well as on author Web sites, as a way of allowing readers to try out the work of unfamiliar writers. The hope is that customers who like what they read will go on to obtain another title for money.