Undoubtedly, the veritable cream filling of the venerable Fighting Fantasy series of books. It is hardly necessary to bother with dice and pencils when reading through this masterfully complex choose your own adventure.
When you come to consciousness at the beginning, you are baffled as to who you are, where, or why. Communicating is a strange endeavor; when you wish to speak, you often kill. When you attempt to help, those you would do well towards flee in terror.
Only slowly is it revealed that you are truly a beast. Your intellect is impaired at first - at the beginning of the book, some decisions you attempt to make (left vs. right) are made by rolling a die. You have enormous strength, you are covered in impenetrable scales. As the book progresses from the labyrinthine dungeon you wake up in out into the world, you slowly unlock mystery after mystery, each layer as to who you are and how you came to be. Your mind improves, you gain greater enlightenment, you are no longer merely a creature of instinct.
The path of revelations that you journey along is truly a metaphysical exploration that transcends all other books of this genre. No other book like this, to my knowledge, is so acute in making one's state of mind so horrific. The alien nature of the character, (you), in a place it does not understand, is the subject of this book, and it is this kinship to the mundane that makes it so compelling.
It is number 24 in the Fighting Fantasy series created by Ian Livingstone and Steve Jackson. The author is Steve Jackson.