"Usted termina de leer este libro."
Translation:You finish reading this book.
The usual answer to the question why is... because it's Spanish.
The de is associated with terminar, so terminar de means stop or finish.
I've said it before, and I'll say it again now. Use a dictionary to see the meanings and frequent compound forms. http://www.wordreference.com/es/en/translation.asp?spen=terminar