Early in the story, Boo is kept shrouded in mystery and intrigue. We suspect that he "appears" in places like the burning of Miss Maudie's house, where he covers Scout with a blanket. We are also given evidence that he is sending trinkets and treasures to the children using the knot hole in the tree until it is sealed up by Nathan Radley, Arthur's father.
When at last we do get to "see" Boo in Chapter 29, his appearance matches the predictions we may have made as readers: He is pale from having been kept indoors, his hair is described as thin and wispy, and he is also described as exceedingly thin or gaunt. The reader, by this time in the novel, should have established or "guesstimated" a mental picture of Boo, and Lee delivers that picture down to the last detail.
Harper Lee uses imagery to describe Boo in chapter 29. "His face was a white as his hands, but for a shadow in his jutting chin. His cheeks were thin to hollowness; his mouth was wide; there were shallow, almost delicate indentations at his temples, adn his gray eyes were so colorless I thought he was blind. His hair was dead and thin, almost feathery on top of his head."
These descriptions model everything we've ever known about Boo. He never goes outside, so he would be pale and his eyes would be lighter in color. If he stays inside, his features would not be considered rugged from hard labor. His features are almost "delicate." All of these descriptions follow what we've assumed about his character all along. Although the ignorance of the stories told about him describe him as a monster, the reader does realize that there must be more to him. How we picture him in our minds materializes before Scout's eyes as she sees him for the first time.