The anticipation of the publication of Go Set a Watchman has led to increased interest in Harper Lee’s other book, To Kill a Mockingbird. One of the strongest backlist titles ever published, sales of HarperCollins’s two trade paperback editions still jumped 226% in the period Feb. 2–July 5, 2015, compared to the similar period in 2014, at outlets that report to Nielsen BookScan. 
Meanwhile, sales of Grand Central Publishing’s mass market paperback rose 60%, selling 209,231 copies in the same period, while HC’s trade editions sold a total of 112,523 copies.

The mass market paperback of Mockingbird has been published continuously since it was first released in 1962. How Grand Central became the publisher of the mass market edition is a reflection of the consolidation that has taken place in the book industry since Mockingbird was first released in hardcover in 1960.

1960
J.B. Lippincott, one of the country’s largest independent publishers, publishes Mockingbird in hardcover.
1962
Lippincott licenses mass market paperback rights to Popular Library, which releases its edition two years after the hardcover.
1970
Fawcett Publications, looking to expand its book publishing arm as a complement to its magazine division, acquires Popular Library.

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