How can I better understand McCullough's book 1776?

Perhaps the best way to understand David McCullough's book 1776 is to understand its division into three parts and to do some background reading on each of the battles that make up these parts. The book is told as a very compelling narrative, but it can be hard to understand the chronology of the historical events McCullough writes about. 

The first part covers the first battles of the Revolutionary War in New England--Lexington and Concord and the battle around Dorchester Heights--and introduces the reader to the leadership style of George Washington and his officers, such as the Quaker Nathanael Greene. Though the British thought they would quickly be victorious, the citizen-soldiers, as the author calls them, in the Continental Army forced the British to lose a surprising number of troops in the battle of Dorchester Heights.

The second section of the book shifts to the battle over New York and the fateful Battle of Brooklyn, also known as the Battle of Long Island, in which Washington's troops were forced to evacuate over the East River and eventually to New Jersey and Pennsylvania. The book's third section deals with the victory of Washington's troops at Trenton and Princeton, which rallied the Continental Army. If you read some brief background information on these major battles before reading the book, you will understand it better.