It sounds like the final disc may have caused Audiobook Builder to crash, which would've prevented the project from saving. The crash could have been due to an intense struggle when trying to read from the disc if it's damaged or dirty. Have you since tried importing the 6th disc again?
If not, perhaps you could try importing just that CD into an empty project, so that if the issue occurs again, other work won't get lost. Also, if you're able to replicate the issue, would you be willing to, just after, generate a System Profiler/Information report and send it over to us at
support@splasm.com?
To create the report, launch /Applications/Utilities/System Profiler.app (or System Information.app, if you're running Lion) and then select "Saveā¦" from the File menu. Attach the report to an email and we'll have a look at it and let you know what we find.