Metered Pay Wall

variety of PayWall that lets someone read some number of pages for free (in a day? in a month? forever?), then forcing payment (Paid Site) for more/full access.

Typically associated with a Newspaper site, but could work for a Book Server also.

Note that this creates an easier opportunity for a "hacker" to defeat your PayWall, but it's worth it. You'll always have to be seeking Copy Right infringers, even from people scanning Printed Book-s.