This is an interesting question. Here is the basic history. To Kill a Mocking Birdwas first published in 1960. The author was Harper Lee and it was an immediate success. It quickly was noted as a great work of literary art and became an instant classic. Therefore, it was no surprise that the book won the Pulitzer Prize.
The book was published by J. B. Lippincott & Co. It was a publishing company that was established in Philadelphia, Pennsylvania in 1836 by Joseph Ballinger Lippincott. Originally, this publishing company published religious material, such as prayer books and other devotional materials. Later it also published medical works, which brought is additional success.
As is the case with many publishing companies, there were megers. One of the mergers was with Haper & Row. Harper & Row still controls the rights of the fiction works, which includes To Kill a Mocking Bird. This is why if you look on Amazon, Harper & Row is the publisher.