Ryuk is the God death who follows you at all times. If you lose the book you will lose your memory. If you run out of pages you may ask the original owner Shinigami for another. 3 rules you must know...

"The human whose name is written in this note shall die."

"This note will not take effect unless the writer has the person's face in their mind when writing his/her name."

"If the cause of death is written within the next 40 seconds of writing the person's name, it will happen."