The first thing that comes to mind is sending them affiliate links to Amazon/Audible, that's pretty straightforward. You could also find authors who are lookin

The first thing that comes to mind is sending them affiliate links to Amazon/Audible, that's pretty straightforward. You could also find authors who are looking to promote their books, and charge them for adding their ad to emails. Also I'm sure there are plenty of software/info-product companies and startups looking for audience in this niche. If you can segment books by niche, it should be even more awesome and profitable. Send programming books and courses to programmers(a lot of them have affiliate programs), business books to business people, etc. If it's not a secret, can you share with us what you did to build this list? The more details the better, it would be incredibly useful!