What is the book of Nehemiah about?
This is a good question and it is important to note that Ezra and Nehemiah constitute one book originally. This point alone will give a great insight. One of the most interesting features of the book is that it does not end with Ezra. One would expect the climax to be the building of the temple, but it is not. To be sure the temple is consecrated, but what is built next is the walls of Jerusalem. More importantly, they too are consecrated.
The main point of the book may be based on this point. The walls of the city constitute an expanded Israel. Israel is no longer a temple community, but a city, to be more precise a new city of God. In other words, there is a expansion that take place.
We also see that God is being faithful to his people. They are brought back from exile.
Nehemiah is about how it's namesake returned to Israel from the court of the king of the Persians and rebuilt the walls of Jerusalem, against great adversity from the surrounding inhabitants. The book is about how Nehemiah built the walls while defending the city against their enemies.