Is this a sentence - Read joke in book?

No, "Read joke in book" is not a sentence. It is a phrase.

Here's why:

* Subject: A sentence needs a subject, the person or thing performing the action. "Read" doesn't tell us who is doing the reading.

* Verb: While "read" is a verb, it needs an object. "Read what?"

* Complete thought: The phrase doesn't convey a complete thought.

Here are some ways to make it a sentence:

* I read a joke in the book.

* He read a joke in the book.

* Read the joke in the book. (This assumes the reader is being addressed directly)