When we go to the library I usually select books to read fairly randomly. Sometimes I have a specific subject in mind, but more often than not I try to come up with ideas for what to do with my children.
Last time at the library I borrowed a book called "An Apple for Harriet Tubman", which describes the life of young Harriet as a slave, and how apples played a role in her quest for freedom.
While reading the story we wanted to find out more about Harriet Tubman's life and found a quiz, and more info at Enchanted Learning.
Then we looked up Underground Railroad, and found this awesome activity on Teacher Scholastic. You can follow a slave on his trip from a plantation to freedom in the North. It is called Underground Railroad - escape from slavery.
You can read or listen to the text, look at pictures/slideshows, and at the end you can even write a coded secret letter. The information is pretty extensive, and we actually had to break it up, because the kids were starting to get a little antsy after a while. But that's the beauty about it. You just stop and come back later.
My daughter was quite upset by some of the things that happened to slaves, and I think it's most definitely one of the most important subjects to teach children. Go check it out.