Write SQL query to solve given problem: What is the average of English books among all books published by Carole Marsh Mysteries?. Keep the solution inside sql tag ```sql [SQL-Query] ```
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'