Any community that only has a mailing list could benefit from experimenting with a proper forum. I've seen this experiment broaden a community time and time again as you move away from only selecting for the type of person who likes mailing lists. And notice that HN isn't a mailing list either.
For example, I would imagine that the sort of people interested in high-quality ebooks extend beyond mailing list loving super-techies. Even a subreddit would be a nice option.
Some people might say it's useful and others might say it encourages people to comment first and possibly focus on polishing the comment second.