Summer in the South is an alternative title for To Kill a Mockingbird.
When giving a book a title, an author usually chooses a theme and bases the title on that. The main themes of the book are coming of age and effects of prejudice. A title that used either one of these themes would be a useful substitute. This title’s use of the words “summer” and “south” are references to each of these themes. Much of the growing up occurs during the summer breaks for Scout and Jem, and the Scout alludes to the prevailing racism.
Somehow, it was hotter then: a black dog suffered on a summer's day; bony mules hitched to Hoover carts flicked flies in the sweltering shade of the live oaks on the square. (ch 1, p. 3)
Titles also often use literary devices, such as alliteration. Alliteration is the repetition of initial sounds. In this case the words “summer” and “south” both begin with the “s” sound.