What is the average of English books among all books published by Carole Marsh Mysteries?
English book refers to language_name = 'English'; 'Carole Marsh Mysteries' is the publisher_name; average = Divide (Count(language_name = 'English'), Count(book_id))
SELECT CAST(SUM(CASE WHEN T1.language_name = 'English' THEN 1 ELSE 0 END) AS REAL) / COUNT(*) FROM book_language AS T1 INNER JOIN book AS T2 ON T1.language_id = T2.language_id INNER JOIN publisher AS T3 ON T3.publisher_id = T2.publisher_id WHERE T3.publisher_name = 'Carole Marsh Mysteries'