Yesterday Apple made available a security update for their iBooks Author App bringing it to version 2.4.1. Full details of this update are available from Apple’s support page for this update. The update addresses an information disclosure issue (in this instance revealing details of the logged in user) that could be exploited by an attacker if you open a specifically crafted iBook Author file.
Since this app is available from Apple’s App Store you should receive a notification to update this app as discussed here. The App Store also offers the ability to automatically install updates (as detailed at the end of the page just linked to). Alternatively, the updated app is available from this page.
If you use and/or have this app installed, please install the update mentioned above to address this security issue as soon as possible.