Apple released iBooks Update 1.0.1 for Mavericks on Thursday. The patch notes are sparse and only mention unspecified, "bug fixes and improvements to performance and stability."
iBooks was brought to OS X for the first time with the release of Mavericks in October. The Mac version of the software includes a different interface than the iPad version, including numerous tools that make it great for studying and textbooks.
The update is a 14.7MB download through the Mac App Store. It has not been added to Apple's Support site as of this writing, but should show up in the next few hours.