I had to move my iTunes Library

and want to be sure I'm doing it right. I have my iPad and my iPhone as devices linked to it == the Library however has lots of missing/dead tracks [not sure why since I don't delete things and am very careful but in any event that's an issue] - I don't mind creating a new Library and actually am doing it now, re-downloading the Apps from the Appstore that I purchased. But before I move music and other items or do anything else, how can I best merge the old Library [which is on an external drive] with the new one that, I hope, is all cleaned up. I have spent some time already backing up all my music etc. to yet another drive just in case - so I have the old Library [which has lots of stuff in it but lots of dead tracks too] to which my iPad and iPhone are connected, the new Library which will at least have the Apps in it but so far nothing else, and the backup of ALL my media, just in case???