http://www.nytimes.com/2016/03/12/business/media/mass-market-edition-of-to-kill-a-mockingbird-to-end.html
According to the email message Hachette sent to booksellers, the license for the mass market edition expired and was not renewed because of “the wishes of the author.” An attachment that accompanied the email said that “there will no longer be a mass market edition of ‘To Kill a Mockingbird’ available from any publisher in the U.S. as per the wishes of the author’s estate.”
The mass market edition costs $8.99, and schools typically get a discount for buying in bulk. More than 20 million copies have been sold.
The edition has been published by Hachette for 40 years. HarperCollins will continue to publish the more expensive trade paperback edition of “To Kill a Mockingbird,” at $14.99 to $16.99, as well as hardcover and e-book editions. The book was first published in hardback in 1960.