How much of a surprise is it to find what Boo Radley really look like has this story before this point prepared the reader for this discovery?

In the book To Kill a Mockingbird it is not really a surprise that Boo Radley protected the children. The reader becomes acquainted with Boo as the story of his family is told by Scout. Boo has a history of having taken scissors and put them into his father's leg. He was kept secluded from the world. The townspeople not knowing the truth could only develop their suspicions.

There is a lot of foreshadowing that gives the reader clues to the real Boo. When the children begin to find small treats and trinkets in the tree, Boo is demonstrating acts of kindness, and his effort to reach out. Jem left his shirt behind after a dare at the Radley yard. It was torn and Boo had tried to sew it up. The book does not tell us directly that Boo was the one who had sewed it, but there are subtle hints.

One of Atticus and the children's friend explained to the children that Boo had been a nice quiet child when he was young, a pleasant child. By putting the clues together the reader is able to preview the likelihood of Boo being a good person.