The security patch released by Microsoft as KB969604 on June 8th 2009 and Office 2007 SP2 has a flaw that corrupts document variables in a document when saved as a .docx file (.docx is the new document file format for Word 2007). This bug affets any application in Word 2007 that uses document variables. The Book Design Wizard and Poetry Book Creator use these document variables to save the formatting options you select when working on your book and reference these variables when you use some of the tools after creating yur book which will no longer work correctly when those document become corrupted.
Microsoft has acknowledged the issue and is working on a fix that will be released in a future update.
Microsoft has released the fix to this issue in a recent update. If you receive this error make sure you are up-to-date with all of the Windows updates on your computer. This will prevent this from happening in any new books you create with the Book Design Wizard.
Unfortunately and old documents that were created and saved as a .docx while the bug in Word was present will still have corrupted variables.