To copy contacts from an address book to a mailing list, use drag and drop in Address Book. To copy contacts from an address book to another address book, use drag and drop, but hold the Ctrl key, otherwise the contacts will be moved, not copied.

Thanks for your answer. It works very poorly. I have tried a number of permutations, and each one fails in some way. If I try to copy ten names, it may just write the first few. if I highlight all the names in the source book, then the list name doesn't highlight when I move my cursor over it - only the other address books do that. I got the closest to success by exporting my entire source address book to a text file, removing the unwanted names by hand, and then importing only a few fields to a new address book - name and email address. When I highlight all the entries in this new address book and drag them to the target list, it copies MOST but not all of the entries. I have never seen drag and drop work so poorly. Any ideas?

If a contact has no email address, then it may not copy into a mailing list, but if this is not the case and copying or moving contacts doesn't work, it may indicate a corrupted address book (target or source, or both). The usual remedy for corrupted address books is to export them and then re-import: http://gsfn.us/t/34o6w

Your partial success with exporting and re-importing suggests this may be relevant.