Media event set for September 9, launch likely ~10 days later.
Apple Releases OS X Mavericks Developer Preview 5 with iBooks App
As noted by MacRumors forum members, the fifth Developer Preview of Mavericks includes both a new beta version of iPhoto (9.4.6) along with the iBooks app for Mac that was first unveiled at WWDC.
OS X Mavericks, which is expected to be released this fall, includes multiple new features like a tabbed Finder, full screen dual monitor capabilities, Safari improvements, and a new Maps app. Developers can receive the update via the Software Update tool in the Mac App Store.
Apple has also simultaneously seeded a new version of OS X 10.8.5 to developers.