What was the first book that Helen read all the way “understandingly”?

This is actually in chapter 21.

I read my first connected story in May, 1887, when I was seven years old, and from that day to this I have devoured everything in the shape of a printed page that has come within the reach of my hungry finger tips. As I have said, I did not study regularly during the early years of my education; nor did I read according to rule.

At first I had only a few books in raised print—"readers" for beginners, a collection of stories for children, and a book about the earth called "Our World." I think that was all; but I read them over and over, until the words were so worn and pressed I could scarcely make them out. Sometimes Miss Sullivan read to me, spelling into my hand little stories and poems that she knew I should understand; but I preferred reading myself to being read to, because I liked to read again and again the things that pleased me.