If you only read one novel, I would suggest To Kill a Mockingbird. The Pulitzer-prize winning book is amazing on so many levels. First of all, it deals with racism in America during the Great Depression. The conflict between whites and blacks in the south is depicted in such a way that readers cannot help but contemplate their own prejudices. The book is filled with richly detailed characters you wish would spring out of the novel and come to life: Atticus Finch, Scout, Jem, and Boo Radley to name a few.
It is the only novel Harper Lee wrote and often it is described as nearly perfect. The book is based on Lee's observations about her father, a southern lawyer. It is filled with scenes that you will remember for life.The novel is told by Scout. Her innocent eyes reflect a world of "What ifs," if only adults behaved better.
The book also was made into a movie starring Gregory Peck. Although so many movies do a horrible job of imagining the book, that is not true with this film. When you read the last page, you will be glad to have another medium to explore with these characters.
I don't read too many books more than once, but To Kill a Mockingbird is one of them.
But don't stop at one. Consider Middlemarch, Atlas Shrugged, Peace like a River, Gone with the Wind, The Heart is a Lonely Hunter, and, well, the list goes on and on.