probably you never had 8gb available user space to begin with
remove them all, check free space then.
subtract 6.4 to see how much the books were using
if you artill curious, add them one at a time, and take measurements
NB dunno for sure but kobo probably use the same sales trick as hard drives where xxGb is calculated using 1000 in place of 1024, thus you never get what you thought you were paying for
see here
https://www.seagate.com/gb/en/suppor...ards-194563en/
if that is in use, then your 8gb is really only ~7.5 to begin with