Such services, different than Book Swapping, allow you to keep a fixed number of books at a time, for a fee.
see www.paperspine.com — www.bookswim.com
"Bookswim, meanwhile, launched about a year ago on much the same idea. Offering more than 200,000 titles, Aberdeen, N.J.-based Bookswim's monthly fees range from USD 14.99 for two books at a time all the way up to USD 35.99 for 11 books at a time. Bookswim hopes to rent out a million books by 2010, representing what it estimates would amount to USD 22,070,000 in subscriber savings over the cost of purchasing those books at list price." (http://www.springwise.com/media_publishing/book_lending_made_convenient/)