Were doing anything in particular when they disappeared? You can try contacting iTunes Support to see if they will let you redownload them again.
Just an update. I sent a message to iTunes who very kindly allowed me too down load the books again
I saved them on my SD card so if they ever disappear again I have them on SD card
thanks again for all replays and I hope this post helps someone else
Thanks again too planb77 for your help