Here's why:
* Missing Verb: The sentence lacks a verb, which is essential to express an action or state of being.
* Incomplete Thought: The phrase doesn't form a complete thought or convey a meaning.
To make a correct sentence, you could use:
* "This book is a thesaurus." (Declarative sentence)
* "Is this book a thesaurus?" (Interrogative sentence)
* "Give me this book, the thesaurus." (Imperative sentence)