Strether, a better way to do it is to load the file into Word, clean it up and then save as RTF and load into Book Designer. if you past in, you lose the benefit of Book Cleaner and somethings are messed up that way because of the lack pre-processing that Book Cleaner does when you load into Book Designer. If you are not using Book Cleaner, have a look here..
https://www.mobileread.com/forums/showthread.php?t=11649