Hello! Thank you for your question. I will be assisting you today.
I'm sorry about the issue. Yes, please delete/move the books you've already read to the Cloud.
When you delete books on the kindle, it will free up space on the kindle.
(In the future, if you want to re-read a book, it's always in the cloud, so you can download/read it again.)
Cloud means online storage on Amazon server (as opposed to physical storage on the kindle.)
So go to Books, then Device tab, then tap & hold on any book you want to remove from the kindle, then remove it:http://www.youtube.com/watch?v=keBLlSJypHE
In the future, you can always go to the Cloud tab, then select any book you want to read/re-read.
Also, check other item, besides Books, such as Music, Videos, etc., and delete.
If needed, connect the USB cable from the kindle to the computer, then on the computer, open the kindle drive/device, then check to see if any data/music/video file is there you can delete.
If all fails, sometimes the device needs to reset. (After resetting, you can redownload any book you want back to the kindle from the Cloud tab, and download any app you need):
Please let me know if there is still any issue. Thank you,
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I thought the 'cloud' was as you have explained but didn't want to do as you suggest without getting the expert advice first in case I lost everything!
I was loathe to remove my books as I generally just skim through them on the first listen or read and them go back to them for a more concentrated experience at a later date.