Not exactly newsletters, but there are 2 interesting podcasts from The Indie Hackers Podcast, one with John O'Nolan and one with Louis Nicholls and it seems like they suggest the best ways of getting people interested in your paid content is just by answering a lot of questions for free on forums like Quora, Reddit, etc. Worth listening to i think anyway, the John O'Nolan one had a couple of good examples of paid content websites that have done well.