If you liked To Kill a Mockingbird, you should look into In Cold Blood by Truman Capote. Its got a similar southern court-case kind of vibe, if that's what appeals to you about Mockingbird. Harper Lee was also involved in Capote's research, which is a nice link between the two and may be of some use in your GCSE if there's any kind of credit for literary context.
The Bluest Eye by Toni Morrison deals with a lot of the same themes as Mockingbird, so it would be worth a punt if you're particularly into the race relations and gender roles in the American south bits of Mockingbird. Fair warning though, its pretty graphic at times. Still, its really good and probably worth powering through even if you're the squeamish type.