JSTOR has launched Books at JSTOR.
Here's an excerpt from the press release:
JSTOR is pleased to announce the launch of its new books program, Books at JSTOR, which brings scholarly monographs from leading university presses and other academic publishers to the JSTOR platform. Books are deeply integrated with the 1,600 current and archival journals on JSTOR. All content is cross-searchable, and books are linked with millions of book reviews and from hundreds of thousands of book citations within the journal literature.