Usually, history books will have subdivisions in each chapter. Divide your summary into these subdivisions. For example, if the chapter in an American History Book was titled, "Freeing the Slaves," there might be these subdivisions:
Slave ownership at the time of the Civil War
Lincoln's Emancipation Proclamation
Amendments to the Constitution
For each subdivision, you could hit the highlights for that section, no more than 5-6 sentences, and then move on. If the subdivision is particularly long, though, devote more time and space to it than the others. End your summary with what you think are the most important points brought up in the chapter.