Which sentence is grammatically correct- or book?

Neither "or book" is grammatically correct. It's missing a subject and verb to make a complete sentence.

To make a grammatically correct sentence, you need to add more information. Here are some examples:

* "Do you want a pen or a book?" (This is a question.)

* "I need a pen or a book." (This is a statement.)

* "He brought me a pen or a book." (This is a statement.)

Let me know what you want to say, and I can help you form a correct sentence!