I'm not a super-user of Moodle so take the following with that in mind...
I've managed to transfer books between courses by first backing up the original course, deselecting everything except the book and course files. Then downloaded the zip file to my desktop, uploaded it to the other course and restored it, selecting the option to add to existing course material rather than delete all existing course material.
Someone else did it this way - http://moodle.org/mod/forum/discuss.php?d=95495
I've had trouble with the import option, so end up doing it the long way.
Try it out on a test course.
Cheers,
Natalie