According to Google, this is primarily meant for online retailers, although the service is available to everyone. Publishers and retailers can now create easy-to-navigate previews of their content which can lead to users purchasing the books. And Google has partnered with several online book retailers to allow Google book search previews of books they sell. Current partners include A1Books and Books-A-Million, with Buy.com and Borders soon to join the partnership (Amazon.com already has a partnership with Google and has had this type of functionality for years, if you were curious).
Google also announced partnerships with the University of California, the University of Texas, aNobii, Macmillan, and a large array of companies and institutions, all of them now allowed to embed book previews into their online catalogs. The number of universities, publishers, and retailers that take advantage of this service is only going to grow as more institutions take advantage of the new API. You can see all of Google's new partners here.
Interested in embedding books like crazy? Try out the preview wizard which will help you (really, really easily) embed some books, like the Mashups book I've embedded below.
GBS_insertEmbeddedViewer('ISBN:159059858X',500,600);