Volo's Monster Manual is a journal passed down from veteran monster hunters. It is a brown covered tome with descriptions and information on various monsters. While you have this book, you will have advantage on Investigation and Perception checks while trying to find signs of or tracks of a monster in this book, and have a lower DC for trying to harvest materials from said creatures. 

In order to add a monster to this book, you must:

• Have met AND Defeated the monster in battle.
• Spent at least 1 hour going over the remains of the monster.
• Take a few hours to add your entry into the book (short or long rest)

Notes:

• The Monster's added for sentient creatures does not tell you all details about them. ex. Having humans does not tell you what every human you track can do. 
• If dealing with a variation of a monster in the book, the DM can decide to give you less information, as said creatures might behave differently.