Correct API GET response when resource is missing: null or 404 not found?

Question

API

For example, there is a GET request to the /books/123 API. There is no book with ID 123.
What is the more correct api answer? Null or, after all, the message not found and the status 404?

Answer 1

If you adhere to the REST API concept, then stackoverflow.com/questions/11746894/what-is-the-p...
Otherwise, it depends on who will receive this data and how to process it.