The Missing Money

Riddle: There once was was a rich man. He walked out of his room and left a twenty dollar bill on the table. When he returned, it was gone. First he asked the maid if she had seen the money. She said she found it and put it under a book. When they looked, the money wasn't there. He then asked the butler. He said he put it between pages 32 and 33. So he opened up the book and looked , but it was not there. However, after looking in the book the rich man now knew who stole the money. Who was the thief? How did he know ?