On page five of To Kill a Mockingbird, the description of Atticus's office is very short and to the point. It says,
"Atticus's office in the courthouse contained little more than a hat rack, a spittoon, a checkerboard and an unsullied Code of Alabama."
Another quote further down the page describes Atticus as,
"He liked Maycomb, he was Maycomb County born and bred; he knew his people, they knew him, and because of Simon Finch's industry, Atticus was related by blood or marriage to nearly every family in the town."
I think by analyzing the two quotes together, we can get a better understanding of what his office reveals about him. First of all, the office is very simple and sparse. This is a characteristic of Atticus as well. Atticus is to the point in his dealings with not only Jem and Scout, but with the other people of Maycomb. He is calm and logical such as during the incident with the lynching mob when he stands guard to protect Tom Robinson. Atticus doesn't take a gun with him to protect Tom but attempts to reason with the mob respectfully. In addition, despite his family heritage, Atticus is a humble man. He is civil to others and knows what it is like to live in another person's skin, a lesson he tries to impress upon Scout. He is neat and simple in his appearance as well as his demeanor.
The hat rack, checkerboard, and spittoon show how Atticus wants to make his clients comfortable when they come to him for law advice. I can see Atticus leaning back in his office chair making his clients feel at ease and accepted in his law office. It's not fancy and only has the necessities (like the spittoon) to do business with the people of Maycomb that he knows so well. The checkerboard is a common social game; however, it also requires strategy, symbolically indicating that strategy is something lawyers need to have in order to win a case.
The unsullied Code of Alabama book also shows Atticus's character. Unsullied means spotless or not dirty. Like the law book, Atticus's reputation and character is not tarnished by his actions or beliefs. He is "clean" in many respects, and that includes his values about racism and justice.
Overall, I think the simplicity of the office shows the humility of Atticus as just a common man. There is no pretense or articles that show that he is better or different than any other citizen in Maycomb.