The University of Michigan Press has started an e-book rental program for over 250 e-books.
Here's an excerpt from the announcement:
Costs will range from 40% of list price for a 30-day rental to 75% of list price for a 180-day rental. For example, a paperback textbook might carry a list price of $22.95; it could be viewed as a rental for $10 for 30 days, or $18 for 180 days. Scholars who rent a book will have the option at the end of the rental period to buy it permanently at an additional charge. Purchased versions include additional interactive features.