I read a lot on my Kindle and use the kindle notes/highlights feature to mark & collect interesting words (generally words I had to look up in the dictionary). So it's certainly a feature that is useful, in the right context. Although for me I guess it's still only a nice to have, rather than something I would seek out. How would you monetise it?
Agree that it's nice to have. I wouldn't die without it myself. Which is why I don't really have plans to monetize it. It'll be a fun project to work on made even better by the knowledge that it's solving a problem for a niche.
