Select IDS where all values can't be found in another table
I think the sentence is quite clear, using a left join to get null USER_ID's and returning those where count = 0; 

SELECT
    BOOK_ID, COUNT(USERS.USER_ID) AS USERS
FROM
    BOOKS
LEFT JOIN 
    USERS
    ON USERS.USER_ID = BOOKS.USER_ID
GROUP BY
    BOOK_ID
HAVING COUNT(USERS.USER_ID) = 0;




BOOK_ID | USERS
------: | ----:
      2 |     0
      3 |     0



db&lt;>fiddle here