The Wizard's Field Guide is granted to the player character by the staff of Hogwarts in Hogwarts Legacy. The book is on loan from the Ministry of Magic with the intention that this book will allow you to catch up to the other students.
You get the field guide in the quest Welcome to Hogwarts by Professor Weasley.
The guide can be accessed via the Pause menu and allows players to see their current Level. Players can also access the following from this screen: