Transferring from iphone to iphone

Hey! Please can someone help me with something that is currently driving me nuts.  Every time I download to Digital Editions, the file will not go onto my current phone but will go onto my old iphone which I need to get rid of.  I subsequently have a load of files that I cannot afford to lose, stuck on my old phone.  Equally I have a file on my current phone that I cannot re-download - there should not be two accounts as I have only registered my one email address but there seems to be some kind of divide - is there a way of safely merging the files so that I don't lose any and can use my current phone going forward?

Thanks so much in advance