I heard that after this season, each book will have 2 seasons
Season 1: Book 1
Season 2: Book 2
Season 3-4: Book 3
Season 5-6: Book 4
Season 6-7: Book 5
Season 8-9: Book 6
This will put them up to 2020. Hopefully the last book will be out by then.
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http://www.backloggery.com/myimmortal