I think that your question is referring to Bud's time spent in the library in Chapter 9. Bud has gone back to the library in order to figure out how far it is to Grand Rapids. Once he knows that information, he can then calculate how long it will take him to walk there. He discovers that it is 120 miles away, and it will take him 24 hours of walking to get there. That's the only information he came to the library to obtain. But before he could leave the library, the librarian handed him a Civil War book. Bud wanted to say that he wasn't interested in history, but he took the book anyway. Bud was pleasantly surprised with the content of the book because it was filled with gory war pictures, which he likes to look at.
I didn't want to tell her that I wasn't really interested in history, it was just that the best gory pictures in the world came from the Civil War. And this book was full of them. It really was a great book.
It's a hilarious line because Bud is judging the quality of a book based on the goriness of its pictures. Regardless of why Bud thinks that the book is good, he becomes so absorbed in the book that he ends up staying in the library until closing time.
There's another thing that's strange about the library, it seems like time flies when you're in one. . . I couldn't believe it, it'd happened again! I'd spent the whole day reading.
Bud seems to think that being in a library causes people to lose track of time. I definitely agree, because I have personally burned way too many hours of my life in places like Barnes and Noble. Of course the coffee shop helps.