When Harper Lee's friendship with Truman Capote was publicized—as well as his identity as the diminutive Dill in To Kill a Mockingbird—many supposed that Capote was in fact the author of Lee's Pulitzer Prize-winning novel. There's enough difference to dismiss this theory, and it's better that way; Capote's ostentation would be ill-suited to the reconstruction of small-town Alabama at the end of the Great Depression.
Besides, the perfection with which Lee captures the character of Scout could only come from Scout herself. The story is by now well-known—the young black Tom Robinson is accused of raping Mayella Ewell, a white girl, and the only man with the integrity to defend the obviously innocent Robinson is Atticus Finch. Finch's son and daughter, Jem and Scout, are brought to new awareness of the world while observing the trial and its consequences.
Part of the brilliance of the novel lies in the fact that this awareness is largely fostered by a figure who barely appears in the story at all. Boo Radley, the neighborhood spook, hovers at the edge of Scout, Jem and Dill's consciousness throughout, but only makes actual appearances once or twice. He's invisible, rejected by the community for reasons he can't even comprehend, reasons for which he is wholly unresponsible.
Prejudice against Tom Robinson is largely the same. No one really thinks he raped Miss Ewell, but the man who did is white, and Tom is black. The bigotry and intolerance peels back layer on layer, no one safe from the suspicion of a neighbor, no one without their own suspicions or mindless hatred, harbored however close.
No one, that is, except Atticus Finch. He emerges one of the truly great figures of American literature, a believable man with a conscience that is nonetheless not above the frailties of the human race. Scout may be the narrator, but Atticus is the central figure, the man who voices and embodies everything Lee wants us to understand, not just about a small Alabama town but about the human race everywhere, at all times.
In the end who wrote the novel scarcely matters. The insight into human loneliness, fear, prejudice, courage and compassion is the equal of most literary works before or since. Lee's beautiful stark prose cuts the narrative to its essential parts, and readers to their souls. There's not much else we could ask for; most of what we get we didn't realize we needed.