How to implement SQL query, max(count(distinct attribute_name))

How to implement SQL query, max(count(distinct attribute_name))

I am trying to display out the I_PUBLISHER with the highest number of
unique author books published, how can I achieved that in an SQL
statement?
Scenario:
Publisher A publishes 10 books with 10 unique author
Publisher B publishes 10 book with that 10 book from same author.
BOOK_ID, BOOK_TITLE, BOOK_AUTHOR_ID, BOOK_PUBLISHER,
SQL statement to get the publisher name with largest number of UNIQUE author