Gives the average number of chapters in Shakespeare's 1599 work.
1599 work refers to Date = '1599'; average number refers to divide(count(chapters.id), count(works.id))
SELECT CAST(COUNT(T1.id) AS REAL) / COUNT(DISTINCT T2.id) FROM chapters AS T1 INNER JOIN works AS T2 ON T1.work_id = T2.id WHERE T2.Date = '1599'