On the home page, there should be two links:
- member login
- guest login
Member help will explain: that you must be someone associated with one of these institutions (link) with an institutional id. Members can: [whatever they can do - view everything, download all PD materials, create bibliographies...]
Guest help will explain: that HT is a member-sponsored db. Guests can search and can view the full text some materials. A guest account allows you to create a persistent bibliography.
On the page for a work, do NOT say: Public domain, Google-digitized. Instead, say what the user needs to know:
Public domain; member-only download.
Public domain; anyone can download.
If you ask for a login at the time of download, ONLY ask for a member login since a guest login does not provide access at this point. The message ("member-only download") may be enough, but the login request could read: requires member login.
This was as far as I got in HT, and I'm not going to be spending much more time there, since as a non-member I am actually served better on other sites. It's a superficial look from a first-time, non-member user.