#5. Your old product is still very valuable
The traditional book cycle has a first "17-weeks" lifecycle where the majority of sales occur in the first 17 weeks. Subscriptions changes this significantly.
Consumption of audiobooks and eBooks has a very long and thick tail.
Focusing on "now available" messaging, related products and merchandising space to aid discovery is very important.
We've not implemented a recommendation system yet, but know that would be very valuable also.