Who caught Becky and saw the book in her hands?

From the text:

At that moment a shadow fell on the page and Tom Sawyer stepped in at the door and caught a glimpse of the picture. Becky snatched at the book to close it, and had the hard luck to tear the pictured page half down the middle. She thrust the volume into the desk, turned the key, and burst out crying with shame and vexation.

"Tom Sawyer, you are just as mean as you can be, to sneak up on a person and look at what they're looking at."