Describe the incident of Mr.Dobbins book: why did he keep it there?

Mr. Dobbins keeps a book locked away in his desk drawer to read when the students aren't around. When Mr. Dobbins leaves the key is his drawer, Becky can't help herself and decides to take a peek at the secret book. She doesn't get any further than the engraving of a naked woman on the front cover before Tom rips the book out of her hand and tears the page. Trouble is in the forecast.... as Mr. Dobbins will soon discover the torn page.