I saw this thread but thought it was worth asking the question again since this question was posted two years ago.
aMember Pro is one we're seriously considering, and also eMember looks really good too. s2, after very brief research, looks more developer-oriented and less well supported than aMem and eMem.
Functional requirements for plugin
1. Access to the online textbook (ideally, available by chapter) after payment through credit card/PayPal
2. Ability to grant coupon or credits for select people to view site (aMember Pro has this built in, but looks just as easily accomplished via eMember's custom fields). We may also just use a separate coupon plugin for this; considering JC Coupon plugin)
3. Ability to create distribution list to email current and potential book users when e.g. sixth edition will be published (Mailchimp Integration)
4. Ability to post and password-protected content for instructors and students
As far as I can tell, both aMember Pro and eMember nicely satisfy these requirements, except for purchasing by chapter only (which I don't think either one does).
Any experiential feedback is welcome. Thank you.