The only way is to save a PDF -
Before ordering your book preview it using this method - http://support.apple.com/kb/HT1040 - and save the resulting PDF for reference - the delivered book will match it.
or duplicate the iPhoto library and remove everything except the keepsake you want and ave tha tiPhoto library - books are not things, they are database entries
Thanks much. This is very helpful. Quick follow-up question -- if I want to save a copy of my iphoto library file to an external hard drive, should I right click on the iphoto library file, create a duplicate and then carry it over to the external hard drive? I do have yet another external hard drive (for time machine), but I would like to save yet another file to a separate hard drive. Thanks again.