Rough number of users signing in at the moment:
~110 / day
~360 / week
~900 / month
Google Analytics RETURNING visitors per month:
~1000 returning uniques
~3800 returning visits
28 min avg visit duration
I'll have a serious think about your pricing advice. I'm plan on increasing the price but removing the free plan scares me. I'm sure it will increase revenue in short term, but don't you think it would seriously harm growth? My current user base was formed largely due to people blogging and sharing on forums and social media. Isn't there a risk this would dry up without a very usable free plan?
I completely agree about sending more emails. It's been on my todo list for a while, I already have Amazon SES set up so I'm planning to roll my own solution to send lifecycle emails to boost engagement. ($50 / month sounds too much, although now I'm tempted to try the free plan just to get an idea of what I'd be missing.)
iPhone app - one thing at a time! I want to polish and optimise the current site first, but I agree an iOS app would be awesome.
Note: Readlang works well on mobiles as a web-app, on both Android and iOS you can bookmark it as a "homescreen app" and it then runs in full screen mode similar to a native app.
wrt to CRM. Try to consider the $50 as an investment. You only need to add X paying subscribers in one month to get a ROI. If you get them, then anything after that is profit. If you don't, then you have learned a nice lesson for only $50.
wrt to pricing - just experiment - take my least aggressive suggestion - allow users to dismiss the notice. Or if you can, split test it.
I think one of the other commenters here is right - you can probably build more of a sense of community around it. Perhaps it can be the place where people really recommend good books for language learners. You already have the voting etc. That then maybe leads you in to the publisher deals, which I don't think would be as difficult as you imagine.