It happens that mailing list subscribers follow the links a little. And it seems that the mailing list is completely dead, no one reads it practically. This impression can be misleading, and if you manage to increase the activity of subscribers, then such a newsletter will become very valuable for its owner.
Write down a specific goal - why you want to increase subscriber activity. For example, to find out if someone reads your newsletter at all. And if there are readers, then sell them a new e-book.
Make a short newsletter issue. Don't post anything. Just ask your subscribers a question related to your goal. Ask if they are interested in the topic on which your eBook is written. Don't tell them anything about the book yet.
Analyze the response. If you receive emails, do not reply to them individually. Instead, publish email excerpts in the next newsletter. And give your comments.
Analyze the response again. If the correspondence with readers continues and you receive letters from other interested subscribers, then everything is in order. You managed to stir them up. Now you can go directly to your goal and offer them a paid product.