Previewed at WWDC, launching in the fall.
Apple Preparing Bug Fix Updates to Mail, iBooks, Safari and Remote Desktop
The site does not reveal any of the changes that are coming, but says they have Apple's standard "improvements to performance and stability" patch notes.
For iBooks, Apple says that the new version (1.0.1) “includes bug fixes and improvements to performance and stability.” iBooks is a new application in Mavericks that allows the reading and downloading of iBooks and iBooks Textbooks.9to5Mac notes that the Mail.app update will come separate from a larger OS X 10.9.1 update and could go live as soon as a few days from now.
Apple is yet to specify to employees what the new Safari version entails, but is labeled as version 7.0.1. The slight version number increase indicates that this is a bug-fix and performance-focused release, not a new version with several new features.