When summarizing anything, it's important to be able to zero in on exactly what's important. To be able to do that, think about the characters that are present, what events take place, and especially what the author is trying to convey, or the author's themes. A theme is any major idea that the author keeps presenting throughout the work. A theme can be a moral or just a simple concept, like pride, injustice, or mercy. Zeroing in on the main themes of the book and even the chapter can especially help you pinpoint exactly what's important in the chapters or work as a whole, enabling you to briefly explain, or summarize, those key points. As we are limited in space, below are a couple of ideas to help get you started.
Chapter 17 takes place just after a stable catches fire. In Chapter 16, when James, one of the grooms of Birtwick Hall, takes Squire Gordon out to visit friends, the stable that Black Beauty and Ginger rest in for the night catches fire. Now, in 17, James is praised for having so bravely been able to rescue both horses, leading them calmly out of the stable, while many other horses weren't so lucky. Through the praise James receives, Chapter 17 especially relays the theme of the need for self-sacrifice. The chapter also relays the theme of the need for human compassion through the conversation between both grooms, John and James, about who will take James's place when he moves on to his new position. John talks about training little Joe Green for the position and relays a moment in his own life when he was orphaned and needing work and was mercifully trained as a stable boy, just like he wants to do for little Joe.
Chapter 18 helps further portray Black Beauty's character, as well as the character of horses in general. Sewell portrays horses like Black Beauty as being devoted workers. Black Beauty gallops his fastest to the doctor's to save Squire Gordon's life and then must gallop back with the doctor. Sewell uses this moment to further dissuade readers from cruelly treating horses, such as using whips or spurs, because, as John phrases it, Black Beauty, or any horse, will "go till he drops" and bids the doctor to "take care of [Black Beauty], sir, if you can; I should not like any harm to come to him." The chapter also serves to raise awareness of how to care for horses because Black Beauty becomes desperately ill when little Joe fails to cover Black Beauty with his thick blanket, thinking that Black Beauty is hot from his heavy running and unable to realize that he'll soon become freezing cold from the exercise and sweat.