I apologize for any inconvenience this problem has caused you.
If you are running Windows 7 then this issue is most likely being caused by Windows location aware printing resetting the default printer during the build process.
We are working on a long term solution to this problem, as a workaround in the meantime you can either disable your network adapter before building a booklet or select the "Always use the same printer as my default printer" option in the Manage Default Printers dialog (you can open this dialog by selecting a printer from Control Panel and clicking the Manage default printers toolbar button).
If you experience any further issues with building booklets can you send a support request through to: support@innovasys.com