Once upon a time, an author had to directly solicit pre-sales for his own book. Those who bought in were subscribing to the book.
From the 1770 book, Antiquity [Google Books link], we see how this worked out in the finished product.
And you would also get a numbered edition, perhaps with an inscription too:
Frankly, I’m exhausted just considering how many times Jonathan Brooks had to pitch his book. Over and over and over again.
And his subscribers wound up having their names displayed two hundred and forty-three years in the future, digitally, on screens around the world.