Building a mailing list is literally the first marketing decision any author should make.
This is because the mailing list is the central element of every author’s marketing strategy. It is the one thing you own and will always be able to control.
Let’s think about it this way: imagine your first book is super successful and sells 10,000 copies in its first month. Now you want to release the second one in the series. How are your original 10,000 customers going to know about it?
If they’re not on your mailing list, they probably won’t.